Administrators can block users and individual IP addresses (or IP ranges; see here). Blocked users or IP addresses will still be able to read the wiki, but won't be able to edit, upload files or add comments (and sometimes, disable talk page access).
Users can be blocked for many reasons. Each wiki may have slightly different blocking policies, but in general, blocks are given for:
Admins are advised to give a warning to people before banning them. It's also advisable to have a working email address in your preferences so you can be contacted by a user affected by a block if necessary. Most blocks should be for short periods, especially if IP-based.
If you feel you have been unfairly blocked, please contact the person who blocked you. Show them the entire message you get when you attempt to edit a page, so that they can make appropriate unblocks if they see fit. A list of admins on each wiki can be found by going to the Special:ListAdmins page on that wiki.
To block a user, admins can use the special page at Special:Block.
Enter the username or IP of the user you want to block.
Select a time for the block. Remember that usually (not always though) blocks are best kept as short as possible. If the timespan you want isn't in the dropdown list, you can type it in the box.
Enter a reason for the block. If a suitable reason isn't in the dropdown, then type it into the box below. Remember that the person blocked (as well as other users) will see this reason, so it helps to make it clear, informative and polite.
An IP block will block all users with that IP, whether they are logged in or not. Because IPs can be shared, you can over-ride this to leave logged in users unaffected by the ban. Check the box marked "Block anonymous users only" to do this.
A side effect of this, is that an anonymous user could create a new account to bypass the ban. So you can counter this by checking the box marked "Prevent account creation".
If a logged in user is banned, then they could evade the ban by logging out and editing anonymously. To prevent this, you can set the block to include the IP that the logged in user is using. Check the box marked "Automatically block the last IP address used by this user, and any subsequent IPs they try to edit from".
In extreme cases, problem users have continued to cause problems via the "email this user" feature. To prevent this, admins can remove this feature for blocked users. Check the box marked "Prevent user from sending e-mail". Warning: It's important that blocked users have some way of discussing their block, and email is often the best way. This box should be used sparingly and only when email is being used abusively.
When you have finished filling in the form, click "block this user" to complete the block.
The form also gives useful links, to allow you to edit the block reasons template, go to the unblock page, and view blocks that are already active.
Frequently Asked Questions about Blocking
How do I block a user across all of Wikia?
If a user or IP is publishing spam or vandalism on your local wiki, and you feel that this may be happening elsewhere and the user or IP may need to be blocked on all Wikia wikis, then please contact Wikia staff or contact the VSTF.
This ability is used sparingly and problems on one wiki don't necessarily mean that a user should be blocked across all of Wikia. In general, this is only used for large scale or persistent vandalism over several wikis, or for impersonations. If there a problem happening only locally, please contact the admins of that wiki, or if there are no admins, see above.
What do I do if I am caught in a global block?
If you are accidentally caught in a global block or were blocked for something via a global block, the first step would be to try to contact the person who blocked you. If that doesn't work, please use Special:Contact to contact Wikia staff. In order to effectively locate the block and look into the matter, you must include all of the information in the box you see while you're blocked, especially the block ID.
What do we do about vandalbots?
A vandalbot is a script which automatically performs some kind of malicious edit or similar operation to a wiki at high rate. Since a vandalbot may be attacking other wikis too, it should be blocked as soon as possible. If you see one, try to contact a VSTF member in the VSTF IRC channel or staff via Special:Contact if nobody is available.