How Do You Delete A Step with the Polygon Lasso Tool

There doesn't appear to be an option for it. The standard advice seems to be to add or subtract from the selection afterward, using other bitmap tools, if needed.

This tutorial covers the Polygon Lasso tool in some depth, and has some decent tips on using zoom shortcuts and "splitting pixels"—as well as using the caps lock key to change the cursor (to better visualize where you click).

http://www.youtube.com/watch?v=VfW_SDxkTOs

Allowing the use of the delete key to move back a step (as in Photoshop) would make a good feature request, which you could submit here:

https://www.adobe.com/cfusion/mmform/index.cfm?name=wishform

Another option that gives you great deal of control is to use the Pen tool instead of the Polygon lasso. Because it's a vector path, you can fine tune the outline very easily using the Pen and Subselection tools, and when you're happy with the result, choose Modify > Convert Path to Marquee. You can even opt to retain or delete the vector path in the Preferences panel.

HTH