I deal with this on occassion with my greyhawk and was looking at the tank and lack of any space above it today. The post by Dancing Queen got me thinking.
If the tank is "siphoning", should it not stop when there is no ability to suck air into the tank? If all water intake valves are closed, then a siphon would create a vacuum (negative pressure) which should stop the siphon (finger over the end of a straw). This seems to be further supported by the post on turning on the pump breifly which would again...break a siphon as suction brings air into the hose.
I may be off base, but it would seem that the siphon then is created by one of the vent hoses, where the vacuum created would have to be offset with air from one of the other two vent hoses.
Perhaps blocking two of the vent hoses is the answer which would be a problem when filling (more volume/pressure in city fill hose than one vent could handle) but should not be a problem when using the pump.