How can you destroy your own units?

You can delete unwanted troops but only when they are wounded. Attack high level zombies with an apc that is underpowered (you can remove the heroes for example) to let troops get wounded and end up in the hospital.

Once they are in the hospital they stop using resources which is good. But you can also remove them permanently. In the 'Heal' screen tap on a troop type icon on the left. This will bring up a 'Dismiss' option. Select how many troops to get rid of and then tap to Dismiss them. The troops will be gone.