Brute force attack is a method of high hardware resources usage in order to decode one’s encrypted sensitive data such as passwords. The application program proceeds all possible combinations of characters in order to find the correct password and login to the server.
As agreed on our Terms of Service, user is responsible for any actions accomplished from or to his server. If your server was affected by a brute force attack, we suggest you re-installing your server and re-creating a strong password. Also, it's strongly advisable to disable direct SSH login and use SSH key to access your server. Please refer to this guide for how to generate and use SSH keys.
For more details in regard to this suspension reason, we suggest reading our Terms of Service.