You can ignore user's if you go to there profile and select the appropriate link. I'd attach a picture to illustrate if it were easy
Basically the post still shows up, but the text is replaced with
*** You are ignoring this user ***
Toggle the display of this post
...with the second line linked to actually toggle.
I noticed you can't ignore "webmaster" though.
It also doesn't hide threads they create from the topic list. Not much of an ignore feature.
In any case, I never bother ignoring (at least via a forum mechanism) users even on forums that allow more robust blocking. If they are truly spamming then they can usually be reported/removed. If they are just annoying, I exercise my scroll wheel.