In a case f a single aspect thaum ritual, you can't just spend one FP to buy out of the whole thing. Which is how every post it making it sound. The aspect being placed is not a compel, everything that happens due to the aspect is a compel.
This is a perfect set up for players to get clever. They either force the target to bleed himself try of FP, or accept the compel. Clever players would try to stage a fight to force FP drain, while focusing entirely on defense. Take off, regroup, and him the big bad knowing he has no FP left and can't fight off the aspect from the ritual.