I have been getting a lot of spam lately. Here's what I'm doing

1) Turn on filtered view on iMessage

2) Actually report the abuse to carriers. iOS makes it easy, but it seems pretty ineffective because the abusers can just use another number. But if you complain to the carrier directly, then they can (hopefully) remove you entirely for that shady customer (and possibly kick them off). Here's what I do

a) Go to https://www.ipqualityscore.com/free-carrier-lookup (or whatever site you like, that's just the one I found)

b) Type in the spam number

c) Find the carrier name

d) Google the carrier, go their site, and find "Report abuse" or something similar

e) Fill out the form. Include your contact info so you actually know whether something is done or not.