MarkB
Legend
I once cast haste on an enemy vampire, then cast a quickened true strike, causing me to lose concentration on haste and prevent the vampire from taking any actions that entire round.
First, Haste only affects willing creatures. Why would an enemy be willing to let you do this to it?
Second, you don't need to do anything special in order to lose concentration deliberately. You can stop concentrating on a spell at any time, even outside your turn, no action required.