How to Delete Old Mentions on Twitterby Alan Sembera
Keeping track of your important Twitter mentions can be difficult when others keep adding your Twitter handle to tweets that aren't really about you. To remove these mentions, your only real recourse is to either permanently block the sender or report the individual tweets as spam or abuse. Blocking the sender also means that you won't be able to see any new tweets from him, and while reporting a tweet as spam removes the tweet without blocking the sender, use this option only as a last resort.
Click the "Connect" tab on your Twitter toolbar, and then select the "Mentions" tab.
Find the mention you want to remove from your list, click "More," and then select "Report Tweet."
Select a reason for reporting the tweet. Select "Spam" if the other person is engaging in spam-like tactics, such as sending unwanted messages or invitations, posting repeatedly about the same thing or if you suspect the tweets are from a spam bot. Select "Abusive" if the tweet is targeting you personally for abuse or harassment, and then fill out a form describing the abuse. Select "Block" if you want to eliminate all the user's old tweets from your mentions list.
Click "Submit" to remove the mention from your list.