The owner of the property has to be on the strata roll. The strata manager can’t refuse to tell you who owns the property, i.e. the person listed as the owner on the strata roll. Taking an example, if there were a breach of a by-law by the owner or occupier of of the lot, then you would have to be able to communicate with them. If the agent is listed as the registered address for the owner, then that is who you serve notices on.
Unless there is specific provision in your by-laws, you can’t tell an owner not to use a specific agent. If they are renting to undesirable tenants you would have to determine if there has been a breach of the by-laws and serve a notice to comply.