vBulletin allows all newly registered users to create and edit signatures by default, which potentially increases the spam in user registrations. Someone would just sign up and put a few spamming backlinks in the signature and leave, without contributing a little bit to the community, probably writing 1 or 2 spamming posts as well. Therefore, allowing signature / signature links does no good to the forum.
To prevent this from happening and encourage users to actively engage in the community, a good approach is to open or enable signature only to those who have had certain amount of posts, say 20; or have passed a certain length of time after registration, say 10 days.
Step 1: Disable signature for “Registered Users”
Go to the AdminCP of your vBulletin, follow the left menu: Usergroups => Usergroup Manager => Edit “Registered Users” by clicking the corresponding “Go” => In the “Signature Permissions” section, select “No” for “Can Use Signatures”. Now all newly registered users will have no signature privileges at all by default.
Step 2: Add a new Usergroup “Acknowledged Users” with signature capabilities
Create a new Usergroup by: Usergroups => Add New Usergroup => Select “Registered Users” for “Create Usergroup Based off of Usergroup” and click “Go” => Name this Usergroup, eg. “Acknowledged Users” => In the “Signature Permissions” section, select “Yes” for “Can Use Signatures”.
Step 3: Set to automatically promote users with over 20 posts from “R” group to “A” group
Now that you have 2 Usergroups with different signature permissions, the last step is to add a promotion strategy that recognizes all users that have 20 posts or more and promotes them from “Registered Users” to “Acknowledged Users”: Usergroups => Promotions => “Add New Promotion” => Select “Registered Users” for “Usergroup”, specify “10” for “Days Registered”, specify “20” for “Posts”, select “Posts” for “Promotion Strategy” and select “Acknowledged Users” for “Move User to Usergroup” => Click “Save”.
Done. Now vBulletin will automatically promote users that have greater than or equal to 20 posts to a new Usergroup that has signature enabled. Other than promoting by posts, you can also promoting users to a privileged group by reputation.
=== Notes ===
In Step 3, you can optionally select “Additional Usergroups” for Promotion Type to add an extra group-ship of the superior group to the user rather than exclusively promoting him to it (so that the Acknowledged Users becomes his only group and he’s not a member of the Registered Users any more).
What happens to users with multiple Usergroups on contradictory permissions?
If a user is assigned to multiple Usergroups, ‘Yes‘ permissions always override ‘No‘ permissions unless one of the groups is a Banned group. In that case, permissions of the Banned group rule – ‘No‘ permissions take precedence.