I've really only ditched 2 people from my group. The first one wasn't showing up all that regularly (often without letting anyone else know) and 1 or 2 of my other players weren't overly fond of him. I just told him that I needed someone that could show up to the majority of sessions. He said that was fair enough but he couldn't guarantee that. It ended up being a relatively mutual parting of ways. I haven't heard from him since (we were acquaintances more than friends) but I think we ended on good terms.
The other one was a friend of mine who I would best describe as a bit of a hermit. He's a little bit strange and lacking in social skills. He wasn't really adding a lot to the game but the main reason I ditched him from the game was his hygiene, or rather his lack of it. We would catch the train to my house together after I finished work. The 5-10 minute car ride from the train station to my house was ok to begin with but started to get worse.
It got so bad that I had to put an air freshner in my car and either wind down my window or put the air conditioner on. It gets worse though. After each game session there would be a layer of crud around the chair where he was sitting. Hair, dirt, bits of food and other unidentifiable objects seemed to form a circle around his seat. To make matters worse, when we first started playing I (stupidly) said that he could stay over when we had a game session and catch the train into town with me in the mornings. (He doesn't have a driver's licence and we normally finish after 11pm on a weeknight).
If he leaves a ring of crud around his chair when he is just sitting in the one spot, you can imagine what it would be like after he had slept in one of our beds. He didn't shower in the mornings before we left (or change clothes, brush his hair, teeth or anything like that), so his smell would be even worse.
In the end, he wasn't contributing to the game at all and me and my wife got sick of cleaning up his crud piles after every session. I took the easy way out here and told him that we weren't playing anymore. We then continued playing as per usual, replacing him with someone else. It's been about 6 months now and I'm happy with my decision.
In hindsight I probably should have just confronted him about the issues I had with his hygiene and poor social skills. However, I really didn't want to have to deal with it. If you can't be bothered showering and wearing clean clothes every day then it is not my responsibility to fix it. Maybe that's not how a real friend should have responded to the situation, but I feel totally at ease with my decision.
Olaf the Stout