There's not much more you can really do, as it sounds like you've got hard surfaces throughout the room, which will yield the strongest reflections. Putting up wood might help, but I don't imagine it'd be much livelier than brick. Taking out any floor coverings, curtains, furniture, or other miscellaneous clutter will increase the reflectivity of the room, for better or worse.
You can also try backing off your mics or putting up some room mics if you haven't already, and blend them in with your close mics. Close micing with cardioid (or a cardioid variant) is going to reject a lot of the reflections coming off the room boundaries, which will take away from the ambience you might be trying to capture.