Try adding another plunger spring. That should slow the action down.

You might also try putting a washer around the plunger to limit the total movement through the coil so that it stops before it is fully extended. It could be plastic or rubber to avoid excessive wear in that area. I like the spring idea that chuckwurt had also.

Make sure the plunger, link and bracket are correct. Shorter link or wrong bracket (with stop) will give you more throw.

Also make sure the part that strikes the ball isn't mushroomed. If it is, get a new one. If it's not, bend it slightly so it hits the ball slightly off center of where it hits now. Doesn't take much to change the kickout. Make sure it doesn't interfere with nearby switch actuator.

It's ok to bend it slightly. You don't need to move it much to get a change. Don't even need a tool. Just bend it slightly using your fingers. You can also try loosening the mounting screws to see if you can get some play there. Just note where it's striking the ball before loosening, then try to tighten it at a slightly different angle. Again, it doesn't take much.

Did you order them using the part numbers in the manual? If not and you aren't the original owner, you can't be sure they're correct. I would try replacing the plunger and link at least.

If loosening the mounting screws for the assembly helps, perhaps washers under the mounting for the assembly will help limit the movement of the kicker arm.

I recently had a similar problem. The kicker would launch the ball up into the glass. I tried bending the arm, but to no effect. It didn't even change the trajectory that much. I added an additional return spring to limit the power. It worked, but it bothered me that I had to do it. After carefully studying the mechanism and how the ball sat in the saucer, I noticed there was a gap between the tip of the kickout arm and the ball when the ball was sitting in the saucer. The result was when the kicker was activated the middle portion of the arm was pushing the ball. The ball was launched up instead of out. Apparently, after years of striking the ball the tip had been bent backwards. I bent the tip forward so it would be in contact with the ball when the ball was resting in the saucer. Removed the extra spring, and the ball now fires forward instead of up. I also regained control of adjusting the trajectory.