This is almost certainly a DDOS attack using an online service. The only way to completely eliminate this is to change to a host that provides hardware DDOS protection. If that is not an option, your best bet is to talk to the person, explain that it is illegal... maybe give them a rank that gives them enough permissions to have fun and feel important, but not enough to do damage. Create a new 'ProHacker' rank, for example, and give them limited perms and responsibility so they play long enough to want to keep it. There's an old saying: keep your friends close and your enemies closer ;-)
Just saying, never use predictable ports unless it's a website. For SSH, the default port is 22, for MySQL it's 3306. If you're still using the default ports, you're just letting everyone know the ports on which you host your administrative services. If your host provides you multiple IPs, set a different IP for accessing system and your server. Also, DDoS attacks cannot be stopped.