You can only use the "boot" icon by using it from a unit next to the unit you want to expel, and only if the enemy unit is a civilian, ie. settler, diplomat, empathic diplomat, slaver etc.
To get rid of enemy military, you must get the other civilsation to sign a withdrawal agreement, which makes them promise to remove thier units from your territory. If they keep wandering back again, download Dale's withdraw script, which forces them to withdraw when they have signed the treaty.
Get it
here .