A good way to avoid an indefinitely freeze by Medusa, is the possibility to kill Medusa with any other creatures with figure, discarding both creature.
It is by the way the official rule cf. FAQ
When a player buys these creatures, they will stay on the board until the beginning of this player's next turn, or until they are discarded by placing another one of these creatures on the same island.
As an example you can hire the Minotaur and place it in the same island that Medusa resulting of the the two creature killing each other. You discard immediately the two creatures.
In addition, it should be forbidden to pay 2 gold upkeep for a creature if you are in Apollo. Indeed you should not be able to use mythological creature with Apollo.