I recently had to ask a player to leave my group because he was just not having any fun, and his lack of enjoyment was bringing everyone else down too.
I managed to do so as politely as possible with the following letter via email. It was discreet, mature and clear. Maybe with some changes you could make use of it.:
I have decided after much consideration, that it would be best if you were to
discontinue playing in my Saturday night game group.
In the past i have been involved in many game groups, and have
experienced the way that small incompatibilites can, if permitted,
blossom into irritations, and eventually into outright direspect,
perhaps even dislike and resentment.
I am resolved to not allowing this to happen. My game group, and the
relationships i have derived from it are invaluable to me. As i am
sure they are to you.
With that in mind i must honestly say that there are small things
which, when taken individually are trivial. My decision is driven by the
awareness that these small things have accumulated over time, and have
become a single issue regarding the group as a whole.
I suspect this may offend you, but it is my sincere hope that this is
not the case. I value your friendship.
I only ask that you do not press me for details, instead having some
small faith that i ask this not out of petty spite or arrogance, but
rather out of a desire to avoid the sort of messy seperations that
lead to lost friendships and ugly game break ups we have both seen in the
past.
I am sure that you will be able to find a group much more to your
liking very soon.
BaldHero
I was surprised at how positive the repsonse to this was. Good luck either way.