Well, better people than me have been trying today to do something about it, without much success, as far as I can tell.
Whilst some users do have html in their usernames to make them underlined, I specifically viewed the pages affected and could clearly see the html at the end of the post. Because it is at the end, it affects everything beneath it, making the poster look innocent.
You can see it on this page:
http://www.theanswerbank.co.uk/ChatterBank/Que stion608468.html
Now click View - Page Source
There is reams and reams of text and html, so click Edit -Find and in the box type
< u >
without spaces. You will see exactly what I mean. It follows on from 'two faced ramplings. welcome back'