There are several contributing factors that occur when a player tops the ball. Primarily it occurs when the player is not staying down and going forward on to the front foot when completing the swing.
Staying down and going on to the front foot when completing the follow through goes against our human nature of trying to lift the ball in the air. The majority of people when given no lessons and are asked to get the ball in the air, they will inherently feel they need to stay back and lift the ball in the air.
In golf we have to go against our instincts and ensure that we transfer our weight on to the front as well feel like we are hitting down on the ball. We have to trust the angle of the club and it's role in making the ball go in the air instead of us helping it.
When giving a lesson and the player identifies that he/she tops the ball, the first aspect I will check on the video is whether the head has drifted on the backswing. This is the number one culprit to why the person tops the ball as well becomes often the solution to the problem.
If I have identified that the head has moved too much on the backswing, I will ask them as an exercise to put their feet together and feel like they are swinging within a phone box. The feeling is as if they are turning in the one spot. As soon as they feel this, the head stops drifting back. I then tell the person to now on the follow through that they move all of their weight on to the front foot and focus on consistently making contact with the ground.
If they do this, they will make a downward strike into the ball rather than staying back and trying to scoop the ball in the air.
If you try this exercise by keeping the feet together and shifting on to the front foot on your downswing, your strikes will improve as well your accuracy and most importantly avoid topping with any club.