On the whole, I have not seen too many trademark decisions involving AOL user names.  My guess is that if someone was using [famous trademark]@aol.com, a letter to AOL might do the trick.  In any event, there has been a Northern District of Texas case in which the owner of the trademark EMC MORTGAGE received a preliminary injunction against use of the email address EMCFORECLOSURE@AOL.COM.  Bear Stearns v. Lavalle, No. 3:0-CV-1900-D (N.D. Texas Dec 3) (not yet up on Judge Fitzwater’s section of the Court’s website).