i see the real flaw as being all the way back at the beginning. "gentleman's agreement' not to use parts of the game is a load of crock. if you can't handle the game, or have ideological differences with how the game was written/designed, then clearly state the rules for your version of the game and state your reasons. if they agree, fine; if they don't, you know they don't want to play that way. if they change their mind later, obviously your play ideology isn't working for them. saying "i won't if you won't" simply invites those that think they can to go ahead. if you didn't want it used, then that's what you should have said in the first place.
so, according to the agreement you made, you are now allowed to use the same spell. that's all. no more, no less. if i were you in such a situation, somehow... i would react accordingly. word spreads. at first, they would dominate. but as word spreads, more people who have grudges against the party will prepare to either use it, or counter it, or reflect it. notable enemies aren't going to be caught flat footed if they can help it. they will be scrying, paying spies, pumping bystanders for information with free ale, and so forth. survivors of the party will want revenge, etc..
therefore, those who want to know what the characters favor as tactics, are going to find out. over time, the tide will naturally turn against the character(s) who spam specific tactics. unless the party leaves no witnesses, in which case, your game has different troubles.