How do I destroy my own terraforming?

Whereas I can vandalize enemy roads/irrigation... how do I delete my own improvements?

It sometimes happens that terrain improvements created by my own settlers/engineers need to be destroyed...
...i.e. because I need to create a "killing area" during war (forcing the enemy to stop because the road is interrupted)
or by swamping a previously drained square (restoring it to the original condition).

(Of course, by editing "rules.txt" I could create a command to swamp a land square otherwise fit for irrigation;
whereas "i" on an already irrigated square creates farmland, I could create "s" for "swamp"... and destroy enemy cultivation...)

BUT

Sometimes I press "F" instead of "R" and see my Engineers create a fortification where I don't need/want one to be... because I am rushing myself and press the wrong key by mistake.

How can I destroy/pillage my own fortifications or roads ?

The pillage function is universal - shift + P - it works the same on enemy improvements and your own.
Of course it can only be done with military units not settlers/engineers.

Once you transform swamp/jungle into grassland, there is no going back - at best, you can plant a forest there by pressing "m" with a settler/engineer.
Grasslands and plains can be forested and also de-forested at will.
Jungles and swamps are one way only, unless you modify rules.txt.

"Jungles and swamps are one way only, unless you modify rules.txt"

My thoughts exactly.

History is full of examples where military leaders modified terrain to their own advantage...

Just as I've seen "settlers drain ocean squares to create islands", I guess I can create a "flood" command

[snicker snicker...]

As for "shift+P", wasn't that for pollution ?

Only 4 terrain types are the ultimate destination of all terraforming: Planes, Grassland, Forest, and Hills. Everything else is one way. If you terraform a glacier, tundra, mountain, desert, jungle, or swamp there is no going back. As already pointed out, pillaging can always be done by your soldiers. As for accidentally making a fort instead of a road, you have two options: if the fort is not made immediately, wake them up and try again next turn. If the unit was precharged and made the fort immediately go back to the auto save at the beginning of the turn.

Shift + P is for pollution when the active unit is a settler or an engineer.
When it's a military unit, its function is pillage but keybinding remains the same.
Compare it to options in "orders" menu at the top menu bar.

OK, thanks