I have someone in my group who is completely new to the hobby. We are now 12 sessions in, and I will most likely need to remove them from the group: Our play styles don't match, and the rest of the group (me as GM + 3 other people) want to play a campaign with a completely different tone than that player - The campaign was "advertised" on Roll20 as a more "serious" game (Story- and character focused), but the player (or his characters, he is on his second one) does nothing but silly/goofy actions and otherwise does not participate most of the time.
They joined the group via "looking for group" on Roll20 (It is an online game), and in the game description I laid out a few requirements/expectations about the tone of the game. The other three players had experience with RPGs so we decided to accept someone who was completely new to the hobby into the group. It quickly became clear that it was a mismatch. I won't go into detail here (not necessary to my question), but they were also below the minimum age I had set for my game (again, in the Roll20 game description on the Looking for Group page) by four or five years.
They received a "warning" from the rest of the group: we spoke with them about the difference in style, and they said they where willing to change their playstyle to better fit - I told them that if they could not (or did not want to), we would probably have to part ways.
Now, a few sessions later, they are returning to their previous behaviour, and the group decided that it was time to "kick them out". Now here is my problem: It is not their fault that the playstyle does not match, they couldn't really know (as they had never played before) and while they omitted their age when applying, we also didn't ask (although we suspected just from their style of writing), and accepted them in. They are also quite enthusiastic, which I really like. I absolutely don't want to destroy their enthusiasm for our hobby, but I am also not that good with people.
So, my question is: How can I make sure that, when I kick them out, they understand and won't lose their enthusiasm? How do I part ways with them "on good terms"?
The player in question is 13 or 14 (not 100% sure), male, the game has been running for about 6 months, with about 2 sessions per month, they received the "warning" around session 5 or 6 and it got better for a few sessions after that.