Do you have a check valve on the discharge line going to the street? This would prevent the backup of water back to the pump making it want to come back on. Things you can try, can you make the holding tank under the sink larger so more water need to be present before it comes on? Also have you tried to put a vacuum break on the system? You can make one easy by razing the outdoor discharge pipe two to three feet high just as it comes out of the house, place a upside down "U" at the top that flows into an open larger pipe that take it to the street. This way if you are getting a back flow you will only get the amount of water between the pump and the upside down "U", however if it was me I would be sure there is a check valve close to the pump so a back flow cannot happen, you can get them at your local hardware or pluming store.
Lastly, the float is adjustable in two locations, one is the location of the float when the maximum water level is reached (turns on the pump) and at the level of water where it turns off the pump, giving a buffer area so if there is flow back it takes more to turn the pump on and less to turn it off, try playing with these two levels on the float.