Why it isnt possible to set city in flames.so if the attacker wants youre city he can start al over
That is heavily frowned upon, and is similar to what I was going to type to the OP before the 502 gateway errors occurred and my post wasn't saved. Funnily enough, he changed his post to something less malicious afterwards.
Don't demolish your city just because you're being attacked, fight for it and put up a good fight and may the better player win. Being a coward and demolishing your whole city achieves nothing except showing immaturity AND you just made it easier for the attacking player since you offered no resistance and all he has to do is spend half a day rebuilding which is nothing (since the demolish times are just as long as the build times).
Learn to defend properly using many methods that are discussed in the forums and enlist help from your alliance and allies. You will learn how to play properly and next time you won't lose the city because you
learnt how to defend it properly, and it will be much more worth your while in the future and whether you come out with the city or not, you have gained valuable BP.
There are other ways to 'sabotage' but I'm obviously not going to discuss it if player's are just using certain game mechanics to be unsportsmanlike.