The best way to disprove the JWs false interpretation of Romans 6:7 is to get them to read and explain for you, 2 Corinthians 5:6-10:
We are therefore always of good courage and know that, while we have our home in the body, we are absent from the Lord, 7 for we are walking by faith, not by sight. 8 But we are of good courage and are well pleased rather to become absent from the body and to make our home with the Lord. 9 Therefore we are also making it our aim that, whether having our home with him or being absent from him, we may be acceptable to him. 10 For we must all be made manifest before the judgment seat of the Christ, that each one may get his award for the things done through the body, according to the things he has practiced, whether it is good or vile.
From the context above, it is very clear that 2 Corinthians 5:10 is suggesting that unfaithful resurrected anointed ones will be judged for "vile things" committed "through the body", i.e. before they died!
We also have Jesus' words at Matthew 11:20-24:
Then he started to reproach the cities in which most of his powerful works had taken place, because they did not repent: 21 “Woe to you, Cho·ra′zin! Woe to you, Beth·sa′i·da! because if the powerful works had taken place in Tyre and Si′don that took place in YOU, they would long ago have repented in sackcloth and ashes. 22 Consequently I say to YOU, It will be more endurable for Tyre and Si′don on Judgment Day than for YOU. 23 And you, Ca·per′na·um, will you perhaps be exalted to heaven? Down to Ha′des you will come; because if the powerful works that took place in you had taken place in Sod′om, it would have remained until this very day. 24 Consequently I say to YOU people, It will be more endurable for the land of Sod′om on Judgment Day than for you.”
Jesus reproached the people's lack of faith in him and indicated that they will be adversely judged for it on Judgment Day. Isn't Judgement Day that future time when the dead are raised up and judged? It surely is, for Jesus also mentions the people of Sodom having a better judgement and we all know that the people of Sodom were long dead in Jesus' day, so logically Jesus was here referring to the future judgment day when the dead will be raised up and judged. So Jesus was clearly suggesting that his first century audience who failed to put faith in him despite seeing his powerful works, will be judged for their lack of faith after they're resurrected at Judgement Day. They will be judged for showing lack of faith in Jesus, before they died!
So I think these two bible passages can be used to reason with JWs to show them that their interpretation of Romans 6:7 is a false interpretation. In fact, a careful reading of the surrounding context of Romans 6:7 and using other translations, makes it clear that the scripture is not referring to acquittal from blame for ones sins, but rather, the body's acquittal from being enslaved to commit sins. The text is simply saying that a dead person is free from the compulsion to sin due to his body being inactive in/paralyzed by death.
