For some reason, I'm personally not bothered when people send you a private message "Hey can you check out my comic?", some people don't like it but I really don't mind. BUT this is just rude. Not care at all for the comic and just write a comment to promote their stuff. Your subs are there to watch your content, it is disrespectful for them too. I would never subscribe o someone who does that.
It is like walking into a restaurant and tell people there "Hey stop eating here, come to my own restaurant".
True, the best is to ignore because your own readers are not even likely to click on their spam.
At least that one offered something in exchange jajajaja. Still is unbecoming to do that.