this is definitely weird. that said, i think your problems would be solved if you just included a note in your toplevel comments that said "no [partner's character] please" or something along those lines - no need to explain your reasoning. i've seen people doing this in the past if they do or don't want to play with certain castmates, and have done it myself to avoid ships i don't like.

if you haven't started the thread yet, then yes, just pm them and be polite and say you're not interested in tagging that character.