depends if you just want to do it manually or through a script.

manually, you just open your bot's IRC folder, look for reaction.mrc, open with wordpad/notepad and remove the line you see that looks the same as the trigger you want removed.

otherwise you'd want to have

/write -ds"on *:TEXT: $+ $2 $+ :#: {" scripts\reaction.mrc

not tested and you would have to add in the rest yourself but that should search the file for the $2 trigger, EG: !reaction-del !slap
(Might need the full pathname on the end, c:\whtever\whateverbotirc\etc\scripts\reaction.mrc.)