Log onto your LMI account and go to MY COMPUTERS. Find the computer you want to delete, and select the 'edit' icon to the right of the computer. From there, you are given 4 (I think) options - one of them being to DELETE the computer from your account.
(DISCLAIMER: I am NOT an employee of LMI, nor do I have any financial concern in LMI. I am strictly a HAPPY USER of LMI products. I am using PC version and XP SP2. Other systems or versions may act different than described above.)