Removing a city
From #openttdcoop wiki
Sometimes when building a station or building the main (or side) line you come across some cities. Sometimes you have to remove them so there is more room for our networks. (Please note that this isn't cheap and only perform this when we more than enough money. Else I would suggest place a sign and wait till there is more money) Removing is done in several steps:
Removing the first buildings
First look at the rating of the authorities of that city. If it's like Very Good or better it's might be smart to remove some buildings like a football station, church and offices. These all need ratings of Very good or better. Removing a simple house will be done later. If the rating is Appaling move to the next step or else remove some houses or something.
If you don't know what kind of building everything is. Use the "Land area information tool". It's that last button in the topside buttons bar (with pause, settings, save, map etc.). If you still don't know if a building can only be removed with Very good rating or higher I would say you remove the buildings that look the most expensive.
I expect your rating is now like Appaling so that means we need to raise that up. We don't use bribe now because that is way too expensive and dangerous. What we do first is demolish everything in a range of something like 10 - 20 squares (please avoid properties like tracks, stations, purchused land etc.). After this is done place trees in the field you just blew up. We don't need fully grown trees, just little ones are enough. Now continue with the next step
Removing some (cheap) buildings
Now your rating is something near Good. This would not give us the abbility to remove everything but at least we can remove roads and houses which a city has a lot of. I found out you can demolish up like 7 - 8 road pieces or 2 houses + 1 road piece a time. I suggest you demolish only houses (and other buildings) untill you can't. Then try to remove some roads. If there are still houses or roads repeat the previous step again. Do this untill there only buildings left that can't be removed.
Removing the last buildings
If it wasn't a very big city , the result you have now must be only a few buildings left. Now it's time for the expensive part (I suggest you save first before doing this because you could be caught): Bribing. To save money, make sure your rating now is 'Good'. If it isn't repeat step 2 again. If you don't think you have enough money purchase every tile around the city (and inside) to avoid them from growing again and come back later when you have enough money.
Now I asume you have enough money to do this. Bribe untill your rating is Excellent and remove some buildings. Repeat this untill there is nothing left. Now purchase the land of the city. This should be like a square of 10 by 10 tiles. Now you have succesfully removed the city.