Hi Michelle, great question!
Any username you see that has 'banned' next to it is usually someone who is only temporarily banned. So, answering them is still OK - they'll automatically be un-banned after their punishment

has been served out.
Anyone who is banned permanently is removed from the site entirely, along with any posts they've made (this is really rare and is usually just carried out on spammers who aren't really VAs anyway)
Most temporary bans are new users who have not read and replied to the Terms of Service prior to posting on the boards as required. Others may be those who have received numerous infractions for things like: overt advertising of their own products or services, trolling for members for other forums or groups, rudeness to other members or staff, or otherwise not following the terms as set forth.
Members on temporary ban are still considered upstanding members of the community who've just had a lapse in judgment or have misunderstood the rules.
Most temporary bans are no more than 7 days, but that all depends on the severity of the issue.
Thanks for asking this question and please do keep right on posting away