We seem to have a lot of posts in Q&A today that would be more helpful to other members in other sections, so if you don't mind I am going to move this to the Habitats & Toys Forum. If you check out that forum, you can do a search for Hemp and read some older posts on the topic as well. (Just don't reply to older posts, of course.)
If I remember right, hemp per se is safe for bunnies. The only issue with any string is if it is ingested and a length of string, not all that long either, gets into their system. It's very difficult for a bunny to digest a length of anything, and you even need to be careful of celery for that reason. So if it's hemp that can't be chewed apart easily, I think it's fine.