Imagine a scenario where player A is the Witch and is in the lead (has the most gold). Player B knows that player A is the witch. On player B's turn, she declares that she is the witch. If no-one challenges, then the ability is activated and player B will swap her gold pile with player A. If player A challenges, then she will win the challenge. When the ability is activated, player A will have to swap her gold pile with another player, but because she is in the lead she will end up with less gold than before.
In this situation, can player A choose not to use the ability?