Removing mailbox/folder in "Mail"

Hi there, I've started using "mail" and I've added my 3 e-mail account, which appear under "inbox" but all my gmail appers again, under "gmail" I want to delete the Gmail folder but it wont let me, I get a message
"the mailbox "(gmail)" could not be deleted, The IMAP command "DELETE" failed with server error: Internal fold cannot be deleted. (failure)"