It is important to keep the ability of being able to alter the non player built roads.
But some kind of penalty should be present to deter user from removing them too much or
for some unintended purpose.
As option, few tile worth of public road could be removed without penalty or small penalty.
Or penalty could slowly increase as player remove more and more. Also penalty could be slowly
reduced as time passes.
One could simply make the price tag higher ... just a line in the pak set.
Why only roads? Realistically, a city would be just as annoyed if you went about deleting its buildings excessively. If I remember correctly, Transport Tycoon has this feature. The town council began disliking you if you removed to much and refused to let you demolish any more. You had to make them happy again (by doing a good job transporting stuff or by bribing them) before you could continue.
I can't say that I have missed that aspect of gameplay in Simutrans. I often demolish a lot of city roads (and some buildings) to build a new and better layout, and I don't want something to interfere with that (the city should be grateful). I could just switch to public player, but swithcing to another player has some drawbacks.
IMO this was a very irritating feature of TTD. Cities wouldn't just refuse to let you demolish any more -- they would also refuse to let you build any stations in the city. I remove a few buildings in order to provide the city with a train station, then I try to place the train station, and then...error! The city won't let you build a train station! Infuriating.
Prohibitive costs are, I think a much better way to go.
I agree with Prissi and Carl - increasing the cost is simpler and makes more sense.
Costs for removing buildings are already not so cheap imho ...
I forgot to mention but the penalty is to be expressed as form of extra cost as
most of people said already.
So it would be normal removal cost + penalty cost.
I think being have to accommodate into the environment will bring more challenge.
So player will calibrate how much one wants to invest versus how much benefit
one will get. Introducing further choosing opportunities.