LOl I know a not so good way to do it..

Write a macro something like /castsequance reset=15 Frostbolt, fostbolt.

Miss spell the second command. It will reset in 15 seconds but the key will become useless because of the miss spelled command.
You will see the red ? until it resets.

I know I know.. poor way of taking a mistake and turn it into what you actually want without another addon to do so!