Board Thread:Support Requests - Community Management/@comment-5578690-20130802192724/@comment-5691231-20130808031411

type "infinite" in the block expiry time bar. indefinite is also an option.

But just so you won't have to keep repeating this, you can install "infinite" through MediaWiki:Ipboptions on your wiki.

If your concerns are about banning a user on chat though, there should already be an "infinite" option installed. If not, install it through MediaWiki:Chat-ban-option-list on your wiki.