Enroute Controller described an airspace infraction after unknowingly disabling the automatic hand off feature when reclaiming the data block after the initial hand off attempt.

Narrative

Aircraft X was level flight; FL320; direct SSO VORTAC. A RADAR associate plugged in and noticed that Aircraft X was direct SSO; routing through White Sands Missle Range. He took Data Block back; flashing to A63; updated route to EWM SSO; then re-flashed to A63. I told him that WSMR was open at and above FL310; took Data Block back; updated route back to direct SSO. Aircraft was close to A63 boundary and needed to process so I decided to wait a couple of minutes before flashing Data Block to A63 so the hand off would not fail. Radar Associate and I were distracted by another situation on opposite end of sector. Both involved aircraft were level flight FL390 and were very much in conflict. It took a while to coordinate between our sector and two other ZZZ sectors to insure separation between these two aircraft. When I looked back to Aircraft X; the aircraft had entered ZZZ2 sector A63 without a hand off. RADAR associate called A63 and they took RADAR on Aircraft X. When I took Data Block back on Aircraft X; it disabled the auto-hand off function. I would recommend making the auto-hand off inhibit function manual only by having to physically having to inhibit auto-hand off.