My b2evolution Version: 3.3

My blogs have been getting various forms of 'ugg boots' spams, either in comments or url. I have reported all the possible urls and they just keep coming with new urls. I want to ban the word 'ugg' but get the following warning:

The keyword «ugg» is too short, it has to be a minimum of 5 characters!

Is there a way to get around it?