Probably easier on you if you don't get DF'd. Question becomes, what is the best way to accomplish that? Answer depends on what they have in the way of Witnesses, what exactly you've told your mother or others, etc.
I would not be inclined to acknowledge they have any authority over you whatsoever. If there were some way to send their letter back as undeliverable with no forwarding address, that would be ideal. Otherwise, I think I'd probably just ignore it. If you talk with your mom, I would not admit that you received any letter.
The whole point here being that you probably have at least some JW family and friends you'd still like to have contact with from time to time. If that is not the case, I don't think what you do matters to much.