Wasps do no harm and do a good job of removing insects

They are probably after the scale insects , you need to keep the ants off and get rid of the scale

As long as you dont attack the wasps they will leave you alone, a good insect to have if you have scale on the tree

The wasps do nothing to keep scale insects under control....absolutely nothing. They are probably after the sweet excretion produced by the scale.

Wasps will, however, control caterpillars if they are present on your tree.

What have you been treating the scale with? Let's see what we can do about that and maybe the wasps will lose interest. Would it be possible for you to take pictures of the scale and the wasps?