No adjustment.
The retaining nut for the idler arm has a washer underneath it (part #12 in the attached diagram). If the nut is tight and the washer is in place the idler arm should not be free to move up and down unless the rubber part of the bushing (part #11) has completely clapped out. The more normal failure is that the rubber fails or the inner part of the bushing sleeve wears which leads to an idler arm that wobbles on its pivot point.
A number of years ago I replaced my idler arm bushing with an original style bushing. It lasted about 4 years (with not a huge amount of use) before it developed a clunk. It appears that the idler arm bushings, like a lot of the motor and transmission mounts 'ain't what they used to be'. Second time around I replaced mine with one of those brass bushings with a grease fitting.
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