I'm going to assume that you have already looked through all of the properties for this control and there is no other propery (ReadOnly, Active, or such) that will do what you want. Try using javascript to dump all of the properties for you, there may be an undocumented property that does that.
If the object has an onChange event then you can save the current value and reset it on every change when it is "disabled". You could also try using onFocus to prevent the control from every getting focus, but I recall trying that and finding that there was no way to 100% block controls from getting focus. You should be able to reset the color of the control to make it look like it is disabled.
The other solution is to put the control into a div and hide the div when the control is disabled. If you want you can create a disabled version of the control that is normally hidden and show it when you hide the active version to create the apperance of a normal disabled control.
Jay