Did I understand you to say that the A/C clutch stays engaged even when you have disconnected the feed connector? If so, then the clutch has seized and must be replaced. If what you are saying is that you disconnected the clutch feed to disengage the clutch, then there are a couple of places you can take a look at. The first question is once you have engaged the clutch with one of the selectors and you then hit the Econ or off button. if the clutch stays engaged, the a relay (M.A.S.) is hung (per Benzmac). Another possibility is that the high pressure side of the system is running above spec. This will cause the clutch to cycle rapidly. It wouldn't hurt to check those refrigerant pressures. Off spec pressures can cause all kinds of weird behavior. Also check the low pressure cutoff switch (located on sight glass block on the receiver/dryer. If this is shorted, this may cause erratic clutch operation. I offer these as possible take a look suggestions. I am sure there are many others out there who know a lot more about A/C than I do. If I were you, I'd be looking for a hung relay.
1987 BMW 325