Ok, I've seen this come up in game time and time again. Someone casts feint, and then tells everyone else not to use it. If you are in a battle against ONE boss, and there are others in the battle with you, you cannot expect everyone to stand by and pass because you have cast a feint that you want to use for yourself. You can ask POLITELY for them to wait until you have used it yourself, but you really can't expect it not to get used. If you don't want others to use it, don't cast it until you are ready. If it still gets used before you can use it yourself, well, that's the game.
On the other hand, if someone has cast Feint, PLEASE don't waste it by casting a wand blast, a small damage over time spell, or a spell that's much lower in rank than what others are casting unless you are the one that cast the Feint. This is especially frustrating when you are fighting a boss with 10,000 health, and people have taken the time to set up boosts for a big hit from a high ranking spell. Sometimes, you just can't tell what boosts have been placed, so I can understand wasting it by accident, just don't don't waste it because you want to see how much damage your bablance beetle can do with it when you've got others there waiting to cast a level 48 spell.
Basically, just be polite and considerate when fighting with a group. Don't waste someone's Feint on a wand blast, and don't expect people to stand around and pass while you get to set up your spell. Be smart, work as a team, and you will defeat the enemy much faster.