replace or remove GPG signature on RPM
Simply (re-) sign the packages with your own key...
rpm --addsign package.rpm
The --addsign option generates and inserts new signatures for each package. Any existing signatures will be discarded.
The --resign option generates and appends signatures for the listed packages while preserving the existing signatures.
If you had imported the public key you can validate the new signature
rpm --checksig -v package.rpm
You should see the phrase Good signature from "Your Name" in the output.
AFAIK removing signatures isn't supported.