I don't know of any way to 'unset' a proc but you could simply overwrite it with a proc of the same name accepting the same number of arguments that simply returns zero (ie. does nothing). Not exactly elegant, but functional in most circumstances I would have thought.
The only problems i envisage is if you use the command [info procs ?pattern?] elsewhere in your code because the proc would still exist, or perhaps if you are deliberately trying to generate and catch an error if the proc doesn't exist.