You can’t block someone from seeing your posts (if you did, they could just log out to see them anyway), but you can ignore their content by visiting their profile and selecting the ignore option here:
This will prevent you from being notified about this person and will hide their posts.
Also, if you’re having a problem with someone on Meta for any reason, feel free to send a message to anyone on the Team if you think it’s something we can help with (admins are listed on About - Discourse Meta).