I've played many games of this and never came across this, but it looks like the rulebook is not clear on this point: If you get a spade, do you have to use it?
There's a few situations in which you might not want to use a spade you got:
1. If you get a spade for a cult bonus, it's possible all the spaces around you are already what you want them to be.
2. You might want to choose power action 6 even if you only need 1 spade instead of 2, to block it.
I cannot believe you'd have to use a bonus if you don't want to.