Algodoo

For this to work requires two simple scripts. One in the box, and the other in the fixjoint. When you click on the box, the fixjoint will be deleted, which will loosen the box from the background or from another geometry that's glued behind it with a fixjoint. In addition to the added scripts, you also need to define a local variable in the box. The local variable is _fixjointID. You create the variable by entering the following line of script in the text entry box in the upper left corner of the box script menu.
_fixjointID := 0 (then hit Enter)

When ready to test it, press the run button, then click on the box and the fixjoint should get deleted.