There are two parts to the answer to your question: one under real estate law and one under divorce law.
Generally, one of two co-owners can force the sale of co-owned property through the process called "sale in lieu of partition." The terms of that sale would be set by a judge, and each party would be able to argue their respective points to the judge about whether or not the occupant should be forced to move out.
However, if in the divorce decree the judge granted one of the parties the "use and possession" of the property, then that decision has already been made. The party granted the use and possession of the property can stay, unless one of the parties go back in front of the Court and explains why the order should be modified. The other party has the right to object, of course.
Either way, the Court will have to be involved to force the occupier out.
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