I expect that if there is control over the wi fi features, then that would include forcing the router to use one particular radio frequency (numbered channel), but I don't know if you can do that on the client end as well - so that Device A will just not see Network B, which is the one that you don't want it to use anyway.
Or if the repeater is expected to give good service, then you could physically block radio signals from the router on a different floor. For instance, standing the router on the lid of a metal biscuit tin ought to stop it from transmitting downwards. Or you could put the client end in the tin, with the open end facing in the direction that you want it to be allowed to see.