If those bad actors in sh.itjust.works and lemmy.world are willing to make an account at a new instance just to send hateful messages on beehaw communities, then it wouldn't matter if kbin.social closes new registrations because they can just go to any other instance that has open registration to send those messages to beehaw communities.
if the issue is popular instances having higher numbers of bad actors, then thats an issue for beehaw/other safe spaces. Rather than an issue for kbin (at least right now). Either way, Kbin is not advertised nor planned to be a safe space (from my understanding of ernest's posts). if there are bad actors then they can voice their hate/opinions and get downvoted/reduced. I don't think anyone on kbin.social cares if there's a minority of bad actors that might "color the site in a bad light". Especially since those bad actors aren't here as of right now.
The solution is for safe spaces like beehaw to continue defederating from popular instances to keep their space "safe". This will continue to happen whether kbin.social closes registration or not.