That's no excuse. An RMA form is something that all their customers are entitled to use. If anyone finds their IP address blocked, even a VPN IP address, then their warranty claim has effectively been blocked for an invalid reason.
The company has failed their warranty obligations.
I think if hosting an RMA form requires paying for 500x more bandwidth than it's supposed to just because it's being hammered by bots on VPNs, and this was your company, you'd block them too.