You cannot control your online communications if you cannot define who can reply to you.
Spam is not limited to people sending you undesired "posts" or images. If anybody or any THING can comment on your existing posts and your software accepts these without question, you have a spam problem. It is not enough to be able to delete or block these after the fact. As we discovered in WordPress blogs in 2002, you can wake up to 30 THOUSAND spam comments and spend an entire day removing them. Then wake up the next morning to even more. When this happens on the fediverse (not if, but WHEN), it is unlikely the network will survive.