Can you disable assassinations in Halo: Reach?

The assassination move (entered by holding melee) is essentially only a taunt as melee from the back kills instantaneously and the assassination takes around ten seconds to complete, not even guaranteeing a kill since the "knife" doesn't hit immediately.

Additionally, a misbehaving controller can cause you to unintentionally enter the stance and lose otherwise guaranteed kills.

Halo 5 clearly has this setting, but I don't know about Reach in the Master Chief Collection.

Halo reach in the masterchief collection does not bring the setting to disable it as it did in halo 5. Reason halo 5 has it is because of mass tournaments that took place with fellow contenders request it to be a added option. I advise getting a new controller since it sounds to me you got sticky buttons.