I've technically fired 3 people before (although they were never serously commited, and in one case hadn't even played with us)....
One I kicked out because he got his hair cut (he's the one who had never played, it was more of a joke than anything else)
One I kicked out because he had something else on when we were practicing, so I gave him an ultimatum, then fired him...... I told him in person, but then left a note in his register to be read out infron of his form (I thought it was funny

)
Our drummer we just never asked him to play with us, so didn't really fire him.... We weren't getting along as musicians (still friends), so just didn't bother keeping him in after he left college.
What you should do is.... Tell him you have a band practice somewhere, set a day and time. Then, on the front of the door to the practice room, pin up a sign saying, "You're out of the band". Then the rest of you don't turn up.... He may see the funny side, and if not, he'll have probably cooled down by the time you see him next.