henry0 - 9:18 pm on Feb 16, 2012 (gmt 0)

Ban or Block:
Yes in two ways,
A) Yours:
-> Login your account -> Go to the profile of a user you wish to block -> Click the user's icon -> Select "Block"

B) By Twitter:
It could happen for a few obvious reasons such as
Spamming, linking to malware, using automated tools to follow, sending over and over the same tweet etc... you get the idea.

Read this:
Twitter Rules [support.twitter.com ]