There are a quite a few things that could cause a block. Bottom line is the path is either too much in to out and/or the club face is open. The big question is which one and why.
There’s a huge difference between a block and a slice. Because most players have an outside in path they hold the clubface open to keep the ball from going left (right for a left hander) this causes the ball to start left and slice. Don’t confuse this as being a block.
A block typically comes from a player who is under the plane, the club coming from way too much inside. This player typically fights a hook, after hitting a lot of hooks they hold onto the release too long and with an inside path and now an open club face they get the block.
This inside path can be caused by a lot of things so for me to provide one drill or check point is tough. I’d lay two alignment sticks down, with the ball between them and have the player focus on trying to swing the club down that “track” and don’t be afraid to release the club face so it squares up.
One last easy thing to check is your eye alignment. Your eye line needs to be parallel to your target line just like your feet, knees, hips and shoulders. If you get your head cocked to the right (right handed golfer) then your eye line will be to the right and typically players swing where they are looking. Put a cap on and set up, then look at the bill of your cap–if the bill is parallel to the target line you’re fine. Or lay a mirror on the ground under your head and look down and see if your eyes are parallel.