I mean, its an option. Unless they are trade cancelling you if which would effect gameplay directly, if you ignore them they will eventually go away.
I do agree that the GMs should do more and have more power, but the GMs for FFXI and FFXIV are dreadfully underpowered and not allowed to do what GMs need to be able to do. SE is so scared of offending someone or of accidently handing out a wrongful ban that they let a lot slide. Believe me I have been playing XI/XIV since 2004, they are horrible at customer service and I am not defending SE by any means, but you are definitely making your aggressor laugh and giving him a thrill by responding. I have had my share of issues as well with nutty players, even ones that came into my estate too just to bother me, but I found over the past couple decades of playing mmos, that if you do nothing back, they get bored super quick and leave you alone. Or take it to a point that they just make themselves out to be really bothersome publicly, which causes more reports from random ppl and more likely of a chance at bans.
But I guarantee you, the fact that this dude knows how bad he is irritating you, is giving him an ever increasing cheap thrill and its only going to get worse if you just keep getting more openly mad and the fact you sent other ppl tells complaining is making his day. You need to take the high road on this one and it will either putter out or escalate to the point that others will notice and send reports too or he will screw up and the GMs hand will be forced. Maybe leave yourself logged in when you afk with the afk timer off, standing in town and let him dig his own hole.
Now if you do not respond and he does try to mess with you on another level like trade cancelling your crafts or teleports or something of that sort, that would be direct gameplay interruption that GMs can actually do something about and almost always do.