Ideally the Registrar module should have a function for this but if not, as a work around, you can add a custom hook per Registrar module to disable showing it client and/or admin side.
Either manually maintain the list or see if the API call can support returning an is_lockable query ( or similar )
Have a look at the EuroDNS WHMCS module to see how they did it.

WHMCS could add this to the Domain Pricing page, similar to the DNS & EPP ,etc, buttons per TLD, add a tick box to enable Domain Lock per TLD, so the option only shows up, no matter what Registrar is in use, but probably better to get whoever developed each Registrar module to add it.