Go to your phpmyadmin and choose the database that you have installed your xenforo forum. Then at the SQL tab run this query:
Code:
UPDATE xf_user_profile
LEFT JOIN xf_user ON (xf_user.user_id = xf_user_profile.user_id)
SET signature = ''
WHERE xf_user.user_group_id = x;
Replace x with the id of the group whose signatures you want to remove. And if you are using another prefix for your tables, replace xf_ with your custom prefix at the query above. Hope it helps.