How to deal w/ a fujoshi in this scenario:
MC: "I'll promise to have gay sex w/ my best friend, but only after you die."
If fujoshi is actually serious about her ship, she'll commit suicide.
MC, after her death, immediately breaks his promise.
This tactic theoretically works for irl shippers in general, not just fujoshis. That said, fujoshis are probably safe from this tactic as long as they stick to fictional bishounens at least one dimension lower than them.