Unless people have played PnP though, and really understand the purpose of the feat they may not even realize its an exploit. ... If someone is found to do this just ask them to stop, explain why, and if they do it again then they are cheating. But please don't place cooldown timers etc on the mode to prevent cheating before it happens
I don't get it. Why do you have to take people aside and lecture them about this tactic that is completely feasible and just looks like "smart gaming" on the engine for someone who doesn't know DnD, is an exploit. But leave the exploit there in the engine to dangle? Such a conversation is seriously a drag for both people involved, and will likely leave the DM burned out, and the player feeling wtf for his good gaming being stomped on.
You can just have the engine remove the exploit, like blindhamster is proposing to do. Since having the AC from CE means your next attack will be at lower to-hit, a 6 second cooldown for toggling CE is the minimum to do to remove the exploit of dropping CE just long enough for your PC to swing, and then clicking it quickly back up again. Hitting that orc with a greatsword, and then quickly toggling CE up, before he and/or his buddies can hit back
is precisely the exploit. To not use the exploit the player would have to keep track himself that if the PC makes an attack (with or without CE), CE will not be toggled on or off for the next 6 seconds.
Everything else is the exploit.
There could be a superior implementation compared to cooldown and spellhooks, though. Just make it do what it should in PnP:
Just toggling CE on doesn't give any bonuses/penalties. When attacking in melee with CE on, the attack is done with a reduced AB, and the PC gains +AC for the next, say, 8 seconds. Is it possible technically?
There's no need for spellhooks, since you'd only get CE bonus when actually meleeing. 8 (or 7 or 6.5) seconds as opposed to 6 so that there's some leeway to make sure CE doesn't have split-second gaps. Of course it'd need a check that (I)CE won't stack with itself or each other. And for several attacks per turn, it'd need the -AB to last as long as the +AC, even if CE is toggled off in between attacks. If the melee-only implementation is done, then the amount of AC from (I)CE could be raised to 4(8) or 5(10), I suppose, too, since flexibility is reduced.