Assuming you are running firmware 2.1.8, you can use the change program (/cp) API to enable or disable an existing program. This is not yet documented in the API document, but it’s in the firmware code:
https://github.com/OpenSprinkler/OpenSprinkler-Firmware/blob/master/server.cpp#L993
for example:
http://x.x.x.x/cp?pw=xxx&pid=x&en=0
will disable the program whose index is given by pid.