Stripping HTML tags and trimming text from webforms

Please help - I need to instantaneously strip HTML tags from webforms as it's typed.

I'm sure I've seen a useful link to this before - a handy facility that enables the coder to instantaneously strip HTML tags from webforms as the info's being entered and also prevent the addition of certain other characters, spaces etc. etc. Really handy to prevent abuse of forums, article posting etc. Any advice gratefully received.