How to ringfence or contain user-submitted HTML

[eluser]Daniel H[/eluser]
Hi all

I allow users to use TinyMCE to submit posts to a forum. While this generally works well I am concerned that if users copy and paste HTML from other sites etc. there is potential for their post to break the structure of the forum; for example if somehow they include a unclosed <strong>, <div>, <p> tag etc.

I still want to allow use of all the major tags, but can anyone suggest a way of ensuring that their HTML is essentially contained so that it won't affect the layout of the page the posts appear in?

Thanks.