You have to be firm about it but not giving them any "apostate" ammunition. Practice doing what The Searcher says in his post.
If they take too long for your liking just do the following -
1. Tell the elders that effective immediately you will not be doing any more platform assignments or leading field service groups. Don't turn up at the meeting on any day you are assigned.
2. Tell everyone else that you have resigned as an elder and are waiting for the elders to sort out the paperwork and the announcement.
That will hurry them along for sure.
just tell everyone you have resigned and are waiting for the elders to get the paperwork sorted so they can announce it.