Gulfstream V N777TY was flown to West Palm Beach (PBI) to undergo maintenance. The airplane was on jacks, for a tire change, when a mechanic needed access to the airplane’s Maintenance Data Acquisition Unit (MDAU) to check out the problem that the airplane was having regarding false over speed warnings. Since the airplane was on jacks the mechanic had to disable the weight-on-wheel (WOW) switches in order to simulate that the WOW was in the ground mode, not in the air mode, and to gain access to the MDAU. The mechanic said he used a “Popsicle stick” to disable these switches. After the maintenance was completed the sticks were not removed, and the inspector that returned the airplane to service was not aware that the WOW switches had been disabled for any reason and no notation was mentioned in the work logs. The aircraft was to be ferried back to Teterboro and shortly after takeoff the landing gear failed to retract. Despite several attempts the crew did not succeed in raising the gear. They elected to land, to evaluate the situation on the ground, and performed an ILS approach back to runway 27R. Shortly before touchdown the power levers were retarded to idle. At that point the spoilers deployed and the aircraft suddenly and abruptly descended to the runway with a very hard, 4.25 g, landing. The right main gear was pushed through the wing causing spilling of fuel on the runway. PROBABLE CAUSE: “Maintenance personnel’s failure to remove wooden sticks from the landing gear weight-on-wheel switches resulting in the airplane remaining in ground mode, and the in flight deployment of the spoilers when the flight crew moved the power levers to idle, during the landing flare, resulting in a hard landing.”