You can do so via chat command. Target them and use: /blacklist add <t>
Alternatively if you want to avoid them noticing you targeting them you can hover their name and go with: /blacklist add <mo>
Both will give you a confirmation box first where you can doublecheck the name. (especially useful for the <mo> variant)

Source: https://eu.finalfantasyxiv.com/lodes...d/7bc2a961456/