According to Clash of Clans' policy, the use of 3rd party software that ruins the fair-play rules can result in a ban. Mods, hacks and bots are examples of software that can lead to banning the account.
If you make any threatening, obscene, defamatory, libelous, racially, sexually, religiously, or otherwise objectionable or offensive comments in the chat, your account might get banned.
If you use your walls or village outlay to write offensive, aggressive or racist words, your account might get permanently disabled. See why Cash of Clans is addictive.
Clash of Clans' policy goes strongly against all kinds of cheating. If you cheat in any possible way, your account might get banned.
According to Clash of Clans' policy, you should have one account per device. Having more than one account, through 3rd party software, can get your account banned.
If you buy 3rd party gems through 3rd party providers, you might get banned. According to Clash of Clans' policy, those services might be using stolen credit cards. See why Clash of Clans is bad for you.
An Inappropriate clan description can get your account banned. Make sure you put a proper clan description that is not offensive in any way.
If you use any kind of PC emulators, your account might get banned. Make sure you use the game properly and avoid the use of any emulators.