How can I stop SpamBots on my server?

Hello there, Kevin here and I'm just wondering about how you can stop SpamBots on my server without any some type of NoCheat or Anti-Cheat plugin because it will bug up my server. What I mean by SpamBots is that there will be bots that are logging in and logging out and there will be mutliple of it attacking my server non-stop. It's like a LoginBot.

How can I fix it? Any plugins to show me how to stop it?