You could send them a letter asking by what authority they are harassing your tenants and warning them that unless they stop, you will take action at NCAT.
If this person is claiming to be the strata manager and, especially if they are taking payments as strata manager, but they are not licensed strata managers, they are breaking the law.
If they have sent any emails claiming to be strata manager, reply asking them to show who appointed them, what qualifications they possess and how much they are charging the scheme for their services.
In the meantime you could apply to Fair Trading for mediation. And ask to see the strata roll which should list the names of all owners. If they haven’t provided that name, they are in breach of strata law and can be fined.
It sounds like someone who doesn’t know even the basics of strata law has appointed themselves to a position to which they are not entitled, not realising that they could get into a lot of trouble for having done so.