They use to make a tool for that. Allen Bar.
Here is a photo of it back when I was given one to test out.

It was just a tube that was pinched on one end and the other end had a handle on it.
A nut driver works too. Slip it over the clamp as shown in the photo and give the handle a tap with a hammer. I liked to use a dead blow hammer.
You can also use a large flat blade screwdriver and place it more towards the inside of the clamp in the “U” shape part if that makes sense.
Hope that helps