Is there any chance to delete a Colony that got a fort? Couldnt find anything online

In-game, you'd have to let the colony starve (depends on the food income of the colony) or antagonize the Indians, leave the colony undefended and hope they raze it to the ground.

If you're willing to edit your savefile, this post at CivFanatics discusses how to use a hex editor to disband a fortified colony, among other stuff: https://forums.civfanatics.com/threads/sav-hacking-disband-stockaded-colonies.71229/

No, according to game rules this is impossible (unless you try the exploits suggested by @ConsulCaesar, although I doubt they will work. Probably using the hex editor is the only solution.)

I believe that starving the colony will work only if the centre tile produces less than 2 food (not possible at all in the easiest difficulty); because otherwise the last colonist would never die of starvation.

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My solution:

If you are willing to go all the way to hex-editing then I assume using the cheat mode is all right too.

With cheats enabled do one of the following two:

- Go into viewing (press V, or right click on a map tile while on the main map), go with the cursor to the colony you want gone (using arrows, numpad, or right click directly on the colony tile)

or

- Go to the menu View / Find Colony, write the name of the colony you want gone and press enter. Same result as the option above.

Then, press SHIFT+D (the regular disband command hotkeys): you will be asked "Really disband [colony name]?", say yes and it'll be gone.

If there are any units outside the colony in the same tile you will be asked first to disband those units one at a time.

Be careful if using this without a previous savegame: when using "cheat-disband" indian villages get deleted immediately (no question) and all the units associated with that village will be gone too.