It may help you some to block media from bot instances, such as BotsIn.space. Of course, that may be inconvenient if you or your users follow bots that post images.

You could also import one of the blocklists from Oliphant or another collection. Of course, their purpose is mainly to avoid bigotry and harassment, but depending on how many of them you're federated with, it may have a slight affect on space used as well.

Of course, none of these are even slightly as effective as deleting media server-side, but that's something the Masto.host owner would have to help you with, here.