How to disable text correction suggestions (underlined) with Google Voice typing?
The best work-around I have found is this:

Tap on the (underlined) word you are interested in changing.
When the (unwanted) suggestion box appears, tap someplace outside of the box.
The box will disappear but the cursor will remain where you originally tapped. 
Now you can either start typing, or deleting, or you can click on a suggestion from the regular suggestion strip below.

Hope this helps.