First you need to properly identify what you have, since control may or may not be needed. People have called the Box Elder Bug a "stink bug" because they are a similar shape but they are not really a pest and do not need control and there are a large number of insects that kind of resemble "stink bugs" many of which, like the Box Elder Bug do not really beed to be controlled. Capture one and put it in a sealed container and take it into your local Cooperative Extension Service office for ID.

Peach trees are realy fragile when it comes to sprays etc, you might do well just to try and pick them off first and dispose of them (wear gloves though).
Make sure its mowed around the tree and remove all weedy areas adjacent to the tree.
If its a major problem as a last resort dust with pyrethrin.

I have nasturtiums planted at my tree bases to attract parasitic wasps which feed on the stink bugs.

Here is a link that might be useful: My veggie home page