Background

Wikidot members can customise their forum signatures via My account, My profile, Forum signature.
However,

8.2. Several users are complaining about the signature of one user, which is promoting a political ideology. The site admin asks the user to change his signature. [...]
Source: WCPG

Admin side

The admin of a site can append some CSS code to his/her custom theme

Hide all forum signatures

.signature{display:none !important;}

Hide pictures (only) in forum signatures

.signatureimg{display:none !important;}

Client side

If you don't want to see forum signatures from a wikidot site that you don't own, first get Stylish addons for Firefox, which lets you customise the way you see any website in the WWW.
The good things with Stylish:

• it's user's choice
• user can later enable/disable signatures with a single click

Click on the Stylish button, Write new style -> For wikidot.com… or For subdomain.wikidot.com… and add one of the following codes:

Hide all forum signatures from community.wikidot.com

@-moz-documentdomain("community.wikidot.com") {.signature{display:none !important;}}

Hide pictures (only) in forum signatures from any wikidot site

@-moz-documentdomain("wikidot.com") {.signatureimg{display:none !important;}}

References

• http://userstyles.org/
• https://addons.mozilla.org/en-US/firefox/addon/2108

• Forums: A step by step guide
• Include a Forum Thread
• CSS: import and use different fonts
• Track Site And Forum Changes In One Page
• Flexible CSS - putting CSS on its own page
• Keep hidden tags really hidden in tags area