Admins are able to block people on a wiki. But also user groups like Staff, VSTF and Helper are able to do.
Go to the contributions of the user you want to block, then click on "block".
When you're on the page arrived you can choose some options for blocking the user like the time of the block and the reason. Click on the "Blok this user" button to block.
Note: Blocking users is not for the fun.
When you want to unblock somebody go to the contributions of the user and click on "unblock" on that page. Now you're going to the page where you can do this action.
You see a user tag on the profile of the blocked user to show users this user is blocked on the wiki. If you want to see the details you can go to the contributions of this user. If you want to see the full list of all blocks on the wiki you can go to "Special:BlockList" on the wiki.