"Web of trust" is an idea talked about a lot, and I think it can go a long way in filtering against spam if implemented well. Coracle utilizes the idea, and so does Nostur. Nostrgram was also utilizing this idea, but unfortunately was abandoned by its developer last year. We each already vet our own follow lists anyway, and if we could decide from there how many additional levels of follows we want to allow into our own web of trust, then we can still have a broad network of other users to interact with while mitigating a lot of spam. For instance, "follows of follows" and so on. As you identify spammy accounts in your own list, you could easily eliminate people them and their network of follows from being part of your own web of trust by simply unfollowing them or even muting them if necessary.