The PROHIBIT constraint is not currently supported for CPLDs. In order to reserve a pin(s) or a macrocell(s) redundant logic has to be used.
In order to keep the Fitter from using the pin or macrocell that you want to reserve for future use, do the following:
Schematic Users -
Drive the pin that you want to reserve with a GND symbol. Make sure that there is an OBUF between the GND symbol and the Output pad. Now put a LOC constraint on this PAD, either in your schematic or using an UCF file. Compile your design normally. In the 'Implemented Equations' section of the Fitter report, you can see that the pin you wanted to reserve is now being driven by GND.
When you want to use this pin in future, just remove the GND symbol and connect your logic to this pin.
The PROHIBIT constraint is available in version 1.5i software.