Honestly I just ignore it :/ I don't like sub4sub because it makes me feel like I didn't earn that subscribers and I didn't and if you don't like it either you shouldn't do it, sub4sub isn't the best idea if YouTube is something you genuinely enjoy
you could always block the words sub4sub in words you don't want in the comment section? That might be the best approach
but whatever you do it'll be fine I wouldn't pay much mind, they're mostly spammers anyways