Did you get their name? Have you still got your BP stub? If you checked in with the agent only (i.e., not online first) then their initials are printed on the bottom of the stub. Write in and complain, making it quite clear you're not after miles/'compensation' (well, I assume you're not - I find if you say this it focuses things a bit better and says you're genuinely aggrieved rather than seeking a handout of miles) but that this is entirely unacceptable and needs to be addressed with the person concerned.