05-12-2013 09:59 - edited 05-12-2013 10:00
1) Highlight the text with your mouse
2) Right click
3) Select 'Remove Message'
A dialogue box will pop up asking for verfication of message deletion to which you simply click 'Remove' and it's done. You can also edit your message in this way.
NOTE to the oblivious: You can only remove/edit your own IM's, not the other person's. ;P