Not Remove, something else...

[wellerpond]

When I am done with a project which involves chatting with new people, I want to remove them from my active list, but keep the option of IMing someone again without having to create the Contact from scratch. Is there an option which allows you to keep a contact, but just not have it active?

[zac]

Move them to another group and close the group using the arrow on the left.

[evands]

Perhaps File->New Chat is what you want? It lets you just type a name, regardless of whether that person is on your list.