It is not open.

Sane networks register prefixes. Other sane networks do not accept prefixes that are not registered to parties announcing them.

If your network does not register prefixes, you need to fire them and move to someone else. If your network does not accept only registered prefixes, you need to fire them and move somewhere else.

Not registering prefixes/filtering based on registrations is having sex without condoms with a toothless hooker behind a dumpster who charges $5 for a full service.