They can't outright refuse to handle DMCA requests, but they can stop automated handling of such requests. If I understand correctly, big content holders get special tools they can use to remove infringing content from some Google services (YouTube at least; not sure about search results). If they abuse that special access, Google can take away these privileges and double check everything manually. That takes a lot of time, and means that the infringing content will remain accessible for a longer time.
I doubt that is legal. That (or something along these lines) would be a good improvement to DMCA process in general, but doubtfully something that Google can decide to do.
How about "request takedown of something you do not own and all your takedown requests will be hand-checked for a month" which will put a reasonable and significant delay into processing. The law doesn't mandate immediate automatic action.
Request take down of something you don't own -> you can't invoke DMCA for a month. That should be a deterrent.