If the whole aircraft carries on spinning after you let go of yaw, then try flicking it into ATTI mode, if it stops yawing instantly, then go and check your gimbal settings. There's a 'Gimbal Yaw in Follow Mode' setting that can cause this behaviour, if it's above 0 then reset it to 0 and see if that solves your problem.
If it's just the gimbal that carries on spinning, then the above won't do anything to help (it only works in gimbal follow mode) and it may be that you need to tinker with the normal gimbal yaw exp values.
Having said all that, dependant on lens length and wind conditions, I've found that the gimbal can exhibit some slight drift at the end of the yaw action. (Most noticeable with the 45mm lens.)