Does God Ever Actively Prevent Repentance?
When we sin, it’s entirely our fault. We’re the rebellious ones. God doesn’t cause it.
Mark 4:11-12 (RSV) And he said to them, “To you has been given the secret of the kingdom of God, but for those outside everything is in parables;  so that they may indeed see but not perceive, and may indeed hear but not understand; lest they should turn again, and be forgiven.”
What at first may seem to us quite strange and “out of character” for God is a common scriptural / Hebraic way of expressing God’s judgment and his providence (while not denying that ultimately men decide their own eternal fates, by either accepting or rejecting God’s grace). Romans 1 explains the dynamic here very well:
Romans 1:18-25 For the wrath of God is revealed from heaven against all ungodliness and wickedness of men who by their wickedness suppress the truth.  For what can be known about God is plain to them, because God has shown it to them.  Ever since the creation of the world his invisible nature, namely, his eternal power and deity, has been clearly perceived in the things that have been made. So they are without excuse;  for although they knew God they did not honor him as God or give thanks to him, but they became futile in their thinking and their senseless minds were darkened.  Claiming to be wise, they became fools,  and exchanged the glory of the immortal God for images resembling mortal man or birds or animals or reptiles.  Therefore God gave them up in the lusts of their hearts to impurity, to the dishonoring of their bodies among themselves,  because they exchanged the truth about God for a lie and worshiped and served the creature rather than the Creator, who is blessed for ever! Amen.
Note that the onus lies upon the people who “suppress the truth” and are engaged in “all ungodliness and wickedness” (1:18). They choose in their own free will to disobey God, then the text says that “God gave them up in the lusts of their hearts to impurity” (1:24). In other words, He didn’t cause their rebellion; He only allowed them in their free choices, to rebel.
The same dynamic is seen in the juxtaposition between Pharaoh freely hardening his heart, which is then (in many passages) applied to God (in a limited sense) doing it: which means that He allowed it, in His providence; He didn’t ordain it.
A fourth example occurs in the book of Job. Satan challenges God to allow him to torment Job. God responds, “Behold, he is in your power; only spare his life” (2:6; cf. 1:12). So it is clear that Satan is behind the direct persecution of Job.
But later, the text refers to “all the evil that the LORD had brought upon him” (42:11); that is, allowed in His providence. Then it is reported that God “restored the fortunes of Job, . . . and the LORD gave Job twice as much as he had before” (42:10) and “blessed the latter days of Job more than his beginning” (42:12).
2 Thessalonians (written by St. Paul, as was Romans) is a fifth example. Men rebel in their wickedness (“they refused to love the truth and so be saved”: 2:10). Then it is stated (as a forceful hyperbolic manifestation of God’s providence and His permissive will) that “God sends upon them a strong delusion, to make them believe what is false, so that all may be condemned who did not believe the truth but had pleasure in unrighteousness” (2:11-12).
It’s not a contradiction. This way of speaking is common in the Bible. When Paul talks about wicked men, he is being literal; but when He talks about God, it is hyperbolic and a form of sarcasm. 2:10 makes it quite clear what caused their damnation: “those who are to perish, because they refused to love the truth.” Even 2:12 again reiterates that man’s rebellion was the cause of the demise of the damned: not because God willed and ordained it from all eternity. The Gospel of John teaches the same thing:
John 12:37-40 Though he had done so many signs before them, yet they did not believe in him;  it was that the word spoken by the prophet Isaiah might be fulfilled: “Lord, who has believed our report, and to whom has the arm of the Lord been revealed?”  Therefore they could not believe. For Isaiah again said,  “He has blinded their eyes and hardened their heart, lest they should see with their eyes and perceive with their heart, and turn for me to heal them.”
The larger context of this passage shows that it is man’s rebellion, not God’s foreordination, that causes the disbelief and wickedness:
John 12:48 He who rejects me and does not receive my sayings has a judge; the word that I have spoken will be his judge on the last day.
Coffman’s Commentaries on the Bible, in its treatment of the Old Testament passage cited in John 12 (Isaiah 6:9-10) states:
In other places the prophecy is referred to as self-accomplished as in Acts 28:27, or as having occurred passively as in Matthew 13:13-15. Here, as Dummelow pointed out, “The result of Isaiah’s preaching is spoken of as if it were the purpose of it.” . . .
The key to understanding lies in the parallel passage of Acts 28:27, which the commentary above describes as “self-accomplished” rebellion. In the overall context of Acts 28, we don’t see the language of God deliberately blinding them, etc. We see their own choices causing these things.
Hence, we see references to “others disbelieved” (28:24); then the Isaiah passage is cited, but in a milder fashion, followed by “Let it be known to you then that this salvation of God has been sent to the Gentiles; they will listen” (28:28). In other words, these hearers would not listen. It was their fault; they were rebellious. God didn’t cause that.