No, you cant block other players (on your datacenter) from being matched with you.

However, if you blacklist that person non of their messages will show up in your chat. Meaning you wont have to see that macro again or hear another word from them ever. You can still be matched with a blacklisted person (anything else would probably break our matchmaking system), but you wont see anything they sent via chat anymore.

For your case that should work out well enough, I reckon? You have a slight chance of having to see him again, but wont see anything he types in that event.

(The only way to really get away from someone would be to switch servers to one outside of the datacenter in question - Ultros is on the Primal Datacenter, so switching to any server thats not on Primal would be an option (at least as long as that person doesnt has characters on the other datacenters aswell). However that might not work forever, since they're working on cross-datacenter connections, apparently.)