signing only certificate?

The free email certificate provided by Comodo is marked as being for signing and for encryption. Is it possible to obtain a certificate that is only for signing?

I don’t want the same certificate to be used for signing and encryption. Can I alter the certificate I have to make it for signing only?

Update: According to the support person with which I communicated, Comodo does not sell signing only certificates. What other company sells signing only certificates? It looks like I may need to take my business elsewhere.