Manual:$wgAllowUserJs

Description
Enables/disables.

This enables a lot of neat customizations, but may increase security risk to users and server load. (Personal scripts can only be edited by the owner and interface administrators, but they may still serve as vectors for various social engineering based attacks.)

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   to 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.