Sanity check for the minimum font size

My board is spammed using the following technique:

Hello, spam!
[size=1][url=http://spam.comLink[/url][/size]

These spam posts often go unnoticed and blend in among regular posts. Even if nobody clicks on the links contained in these posts, it builds backlinks and pagerank for shady websites.

My suggestion is to set a reasonable minimum font size so that the spambot links would be visible. Nobody needs [size=1] anyways