Does God punish Christians for sin?
Yes.
He does.
In some cases He'll "take them out".
Like He did with Ananias and Sapphira.
We don't have enough information to state conclusively that Ananias and Saphira were genuinely converted to Christ in their hearts. Many people join the Church to be religious, but come short of making a true heart-felt commitment to Christ.
I agree with the previous posts that all sin has been forgiven, and that there is no further punishment for sin for the truly committed Christian. This is because Jesus is a complete Saviour, not a partial one.
If we go by the parable of the wheat and the tares, then we can apply it to our churches, because there is a mixture of religious and genuine Christians in our churches. But the foundation of God stands sure, God knows who are truly His, and on the Day of Judgment, the true believer in Christ will be separated from those who are motivated by just a religious spirit, and the Scriptural picture of the sheep being placed at the right hand of God and the goats at the left is of the separation between the true believer and the religious hypocrite.
Jesus said that trying to pull out the tares from the wheat before the Day of the Lord will damage the true wheat. This has happened in our churches when judgmental and legalistic people have tried to separate who they thought were the hypocrites and have harmed good believers instead. The problem with religious hypocrisy is that these people are so like true believers that it is impossible to tell them apart. This is because God can do what no man or woman can do: look into a person's heart. So He saw what was really in the heart of Ananias and Saphira, saw their hypocrisy, and dealt with them accordingly. Up until that time, no one, even the Apostles themselves knew that these two were hypocrites. Peter only found out through having this revealed to Him by the Holy Spirit as a word of knowledge. He could not have known otherwise.
But that is not an excuse for judgmental folk to misuse the Word of Knowledge to root out what they believe are hypocrites in the church. That misuse of the gift has caused widespread suffering, heartbreak and harm to good believers.
We need to take notice of what Paul said about himself. He would not allow any other person to judge him in the way he conducted himself, and furthermore, he would not even judge himself, but leave the judgment to his Lord and Saviour Jesus Christ.
If we are prepared to leave the judgment of people to the Lord, then as the Scripture says, their sin will find them out, if not in this life, certainly in the Judgment.