it's not all or nothing. you can let them go now and vest them 6 months. i would keep in mind, this person may not have been productive, but they have taught you an extremely valuable lesson. you should vest them at least the 6 months.
Let me give you a real data point of what happened at a leading social network company. One of the early hires was fired after a few months on the job, and the company agreed to not only vest him, but FULLY ACCELERATED, 4 years. Of course lawyers drew up non-disparagement agreements and such. This actually happened multiple times over the course to IPO, but it was a smart move in hindsight to ensure there was no leaks and legal battles that could cause huge problems later.
it sounds like your company is still small. i wouldn't decide by committee, but do be transparent with everyone after you make the decision.