Report and block, all we can do.

A trailing space is just one of many, many ways they can make a name that looks the same. Nostr clients support Unicode AFAIK, so there are many alphabets to choose substitute letters from.

Nostr relies on people rather than centralised authority to defeat identity theft. Well, there is also NIP-05 for validating your npub against external identities, for public figures etc. But mostly trust your friends to report and block the fakes.