Many forums have a problem with people breaking signature rules. So to be able to give them warning points that always meet the thresold for the signature-cutoff can be a good idea to temporarly disable the user's signature.
However, sometimes administrators dont want to disable their signature when giving other warning types and points.
How about;
1) Have the signature-disabled threshold level as it is now
2) Also include the option when adding a warning type to "Kill Signature" or "Kill Avatar" ("with this warning") - and when this specific warning matures, they can use the signature again