user name censoring

I have a few words in the user name censoring, including the s**t word with an asterix at the end. Tonight some one has joined my forum with the user name sickaspigs**t (no asterixes) Okay I have had worse, but how do stop people using s**t or other words I don't want, as part of their user name?

since the astrix was at the end of the word, it did not stop the person from putting stuff before it try using
*Shit
AND
Shit*
AND
Shit

to block them all

Thanks for your reply kinky I will give it a go

Alright.