If you enter several forbidden words as username, you have to seperate them by comma. However usually you put a space after the comma. (I think this is absolutely standard).
If you do that in this field, the entrys will no longer be recognized hence the forbidden words filter will not take effect.

I would like to see a routine, which checks the entered words if there is a leading space (or a following space after a comma) and applys the word filter the same like if there was no space entered.