I like queries.
Change the
red piece to specify the group id that grants signature permissions.
Backup first. And please understand that this clears signatures for all users who do not belong to the specified group. For example, if an admin is not also in that group then their signature will get cleared. You may wish to exclude admins and mods, like this:
Rich (BB code):
UPDATE xf_user_profile AS user_profile
SET user_profile.signature = ''
WHERE (
SELECT user.user_id
FROM xf_user AS user
WHERE user.user_id = user_profile.user_id
AND NOT FIND_IN_SET(23, user.secondary_group_ids)
AND NOT FIND_IN_SET(3, user.secondary_group_ids)
AND NOT FIND_IN_SET(4, user.secondary_group_ids)
);
Basically you are specifying all secondary groups that are allowed to have signatures.