Here's one option:

First go through your existing contact roles and manually grant permission to send you posts in each one of them. Disregard that this permission is inherited from the channel role. Soon it will no longer be, and we don't want there to be any disruption, right?

Next, change your channel role to Custom if you haven't already done that. Adjust your channel role as required.

But, and here comes the important part: Grant permission to send you posts only to those whom you explicitly allow.

Next step: Make at least one new contact role that matches whatever contact role the spammers have. This time, however, don't grant them permission to send you posts.

Finally: Assign this new contact role to the spammers.

Enjoy the silence. They can still send you DMs, you still receive their comments in both your own and other people's conversations, but they can no longer bomb you with posts.

If it's only repeats (boosts) that they spam you with, it's easier:

Activate the option to have filters per contact.

Then go to everyone who spams you with repeats and add the following to the blacklist:
?verb == Announce

This keeps repeats from these contacts out.