I mean, as long you're forcing spammers to break link formatting to send users to a malicious domain, there's not really any reason to worry IMO. If the spam text isn't enough on its own to tip off normal users, seeing something like "some-random-site DOT net" on a site like this should make it more obvious. As for blocking specific domains, it'd probably make sense to block TLDs by default unless on a whitelist, then apply a more specific blacklist if needed.