I am only using two registrars and wanted to get rid of all the other ones in the drop downs when doing a manual registration/transfer of a domain. This also applies to all modules, including payment gateways and such.

1. Go to your /whmcs/modules directory and make a new directory called registrars_unused
2. Now move all the registrar folders in /whmcs/modules/registrars that you are not using into that directory.
3. Voila! No more extra registrars in the dropdown.
4. If you need to readd a registrar, just move it out of the registrars_unused directory and back into the registrars directory