I do some self promotion, although I try to; 1: only post in relevant forums. 2: also engage outside of my own posts an then not talk about said posts.(this time is an exception but its kind of a meta discussion.)
Due to the nature of the fedivers what I post on mastodon and pixelfed might get federated on to Lemmy, which unfortunately might give the appearance of spamming, I'm aware of this but I don't see a way around that. And lastly. I don't think my videos on Peertube would get any views unless I post them outside of peertube since there is no real algorithm or good discovery functions across the instances. But I'm open to being in the wrong here.
I'm not saying it will be the solution but, positive reinforcement for those who behave, mixed with not interacting with those that only spam would probably get some to loose interest. And perhaps some mod rules.