To keep fairness in Pokemon Go, Niantic bans accounts doing dishonest activities. This page gives you the details of account banning: the reason why an account is banned, howto avoid getting an account banned, and how to recover an account banned mistakenly.
Dishonest acts subject to a ban
Here are examples of behavior which are subject to a penalty.
Using location spoofer, fake GPS, and other cheats/hacks
Buying, selling, and trading a Pokemon Go account
Sharing a Pokemon Go account with other users
Having multiple accounts
Connecting a Pokemon Go account to tools developed by third party
On the official website, Niantic notes about cheating, hacking, and dishonesty as below.
Your account was permanently terminated for violations of the Pokémon GO Terms of Service. This includes, but is not limited to: falsifying your location, using emulators, modified or unofficial software and/or accessing Pokémon GO clients or backends in an unauthorized manner including through the use of third party software.
The table below shows the popular unofficial tools. These helps players see individual value of Pokemon, find Pokemon location, and do other activities in Pokemon Go. Some of those tools are already unavailable, such as PokeVision, due to Niantic's argument: those tools are same to cheating and hacking.
To use some of those tools, players have to connect their Pokemon Go account to tools, such as Pokemon Go Nexus. Since that behavior is subject to a ban, using tools requiring account connection is a player's own responsibility.
What happens when a player does cheating, hacking, and other dishonest activities are not exactly revealed. An account may be closed or assigned punishments. The followings are penalties considered as being confirmed.
Unavailability of PokeStops
A player cannot get items from PokeStops.
Unavailability of gyms
A player cannot put defenders in friendly gyms and challenge rival gyms.
Unavailability of catching Pokemon
A player cannot capture a wild Pokemon。
If an account is closed for account banning, a player cannot play the app.
Failed to get game data from the server
When the message "Failed to get data from the server" appears, the account is possibly banned for a dishonesty.
How to recover a banned accoount
As Niantic said as below, a player can send a request to make a banned account available to use again if the account banning is failure. Additionally, a player may be able to solve penalties by logging in little later.
If you believe your account has been terminated in error incorrectly, you can appeal that termination through this form. For privacy reasons, please do not post appeals on social media.