Shower leaking

We purchased an old house that has what appears to be a custom shower stall, no tub. There are 2 knobs (1 hot, 1 cold) and a shower head protruding from the wall -- no name brands on anything. The problem, if we just turn the knobs off manually - the showerhead still flows water, we have to use a wrench to turn the knobs further to get the showerhead to stop flowing water after we shower. Is there anything we can do to repair?

The valve body can usually be replaced. You might also need to replace the valve seat (if it is a replacable type). A picture would help. Normally, if you take the handle off (usually a screw behind a decorative cap you pry off the front) you can then remove the decorative trim and then take the valve out. Take it to a good plumbing supply house, and they can usually match it up with a viable replacement. The big box stores carry the more common ones, but sometimes the generic replacements aren't the best quality.