Sure. Anybody with the correct information may cancel a ticket (bookings, PNR's and the like are irrelevant).

The question is whether there will be a refund and unless the tickets are refundable, the tickets won't be refunded either to the TA or to you. This leaves the TA doing this merely out of spite.

Is there anything which leads you to believe that the TA doesn't like you so much that he is willing to do this to you for no particular reason?