C208 pilot reported the autopilot engaged uncommanded several times. Pilot kept the disconnect button depressed for the remainder of the flight until landing to prevent the autopilot from engaging.

Narrative

On the evening of Date I was operating on C208B from ZZZ to ZZZ1. Weather was IFR in ZZZ1 so I was departing on an IFR flight plan. I spent around 5 minutes on the ground from startup to takeoff to ensure that all GPS's were fully booted up; avionics were all set; and autopilot had completed all of its self tests. My plan after takeoff was to hand fly the aircraft to 400 feet MSL; accelerating to 95 knots; then retract the first 10 degrees of flaps and make a climbing turn towards ZZZ1. The takeoff roll was normal; and I believe I rotated at roughly 60 knots. Around 200 - 300 feet MSL I had reached 95 knots and was preparing to retract the flaps and begin my turn when I heard 'Engaging Autopilot' spoken over my headset. I felt the autopilot servos engaging; the aircraft began to pitch down; and the 'LVL' autopilot mode was displayed on my PFD. I pressed the autopilot disconnect button; which seemed to disconnect the autopilot long enough for me to retract flaps and begin my turn. At this point I thought I had accidentally engaged the autopilot in some fashion and was quickly trying to determine what I had done. Within a few seconds of my pressing the disconnect button; I felt the autopilot engage again with no action from me. I pressed the autopilot disconnect a second time and felt the autopilot disengage; then reengage and continue to attempt to pitch the aircraft nose down. At this point I pressed and held down the autopilot disconnect button and continued to for the remainder of the flight; which did fully disconnect the autopilot.After these two engagements with no prompting from me I made the decision to return to ZZZ immediately. I radioed Tower and informed them of my need to return to the field; and that I could return VFR/ accept a special VFR clearance if possible. Flight visibility was fine and I had not yet entered the clouds. Tower cleared me to land and I flew a visual pattern back to the runway. On a base leg I did release the autopilot disconnect button momentarily and the autopilot did not reengage; however I elected to press and hold it down until I landed in case of another uncommanded engagement. The landing was uneventful and I taxied back to the ramp. Upon shutdown and postflight no circuit breakers were found to be popped.An unknown system issue with the GFC 600 autopilot which led to repeated uncommanded autopilot activation. If the issue was caused by a failure of the GFC 600 unit; that issue needs to be fixed. If it was due to a lack of knowledge of the GFC 600 system and its failure modes; perhaps more thorough training on the system is required. I did consider pulling the autopilot circuit breaker to reduce my workload during the flight; however due to the fact that my arm on that side was occupied flying the airplane and holding the autopilot disconnect; the phase of flight; and the weather I felt it would have taken too much of my time and attention to do so. Collars installed on important circuit breakers such as this one to make it possible to pull them by feel or with a quick glance might help situations such as these in the future.