There are two broad solutions to the question of free will existing in heaven, namely, that free will is altered to allow only good decisions, or that free will remains intact but is never used for evil. Those people arguing in favor of the latter solution suggest that the beatific vision that accompanies one's entry into heaven removes all desire for evil.
Each of these solutions theoretically solves the theological problem of there being no sin in heaven while, at the same time, retaining some allowance for free will. However, if the former solution is to be accepted, it raises a number of questions. Among these is why, if God can alter free will in such a way, he did not do so in the first place.