Manual:$wgAllowUserJs

Description
Enables/disables user JavaScript pages.

This enables a lot of neat customizations, but may increase security risk to users and server load.

A related setting   allows/disallows users to use their own CSS files.

Usage
To enable users to perform their own JavaScript, append the following line to your  file:

To disable this function set $wgAllowUserJs false or delete whole line.

Important note
Microsoft Internet Explorer and Mozilla/Firefox allow the embedding of JavaScript into stylesheets using HTC or XBL.

So in order to prevent users from defining JavaScript, you need to disable   as well.