Well, part of the point of the GPL (under which the official client is licensed) is that you can't really prevent people from doing that. Of course the party operating the servers has the discretion to ban users of modified clients on sight, but recognising those users may be difficult. The GPL forces them to provide sources with any new release of the official client, and barring some sort of endpoint integrity attestation in the spirit of DRM systems in games (which a far larger segment of their target group than those who would use unofficial clients would reject) they could not distinguish a patched build whose behaviour on the network is the same as the official client from their own.

I think any "homesteading the noosphere"-style arguments that grant Moxie moral rights to control the use of the software that go beyond his contractual rights are rendered much less compelling by the presence of network effects. To interface with Moxie's captive userbase, you can't avoid trespassing on his homestead.

Moxie has made his decision; now let him enforce it.

Refer to other comments - this isn't really a "third party client," it's an exact copy of their code with two or three lines changed.

Molly is a hardened Signal fork without the crypto stuff