Your travel agent could not have 'deboarded' you. They could only have cancelled the reservation and from what you say it appears that this was not the case. Even if they cancelled your reservation after the boarding pass was issued, the cancellation in the reservation system is unlikely to go down into the DCS system and to the gate agent. This depends on the degree of synchronisation between the reservation (distribution) system and the DCS. BA use integrated systems from Amadeus for both but I doubt if they are synched to this degree.
Generally a change of e-ticket coupon status can only be performed by a carrier's control desk and such a change after BP issuance would be unusual in the extreme.
Conclusion, as davidjohnson6 says :
it sounds like a clear case of denied boarding, for which standard compensation applies.