It could absolutely be done through a client, but to truly suppress and censor on Nostr it would need to be done on all the clients. I think if users realized their client was doing this they would move to another and developers would build new ones that are uncompromised.
Discussion
Exactly. Competition protects consumers better than any government. If one client or relay starts to censor they would lose their users very quickly to their competitors.
As long as your connected with other relays they could censor what you've written or posted?
No, every relay would need to censor.
Every relay you are connected to I should say.
No, a single relay could only censor what you have written to all the users which have only this one relay in common with you. All other users who have more relays in common with you can still see everything.