I'm not totally sure what you mean by saying that a particular IP is slamming you but using many different IPs. Unless you meant IPs owned by OVH?
If so, you could potentially ban a significant portion of the OVH range -- it's fairly unlikely that legit users will register there but there's always that possibility of a false positive.
There are add-ons that may help. Another alternative is to create a required custom field that requires a value in a particular format. Both of these serve to simply make your forum more unique and thus not as easy to target (given that XenForo is a high value target, but your specific site is not).
Note that if you setup a StopForumSpam API key, you'll feed back to SFS and future registrations from that IP (for example) will be more likely blocked.