I had similar issues with spam bots. Best defense is both a honeypot channel and cross channel spam prevention within a certain time bracket, about 1.3 seconds to 8.6 seconds is the sweetspot. The lower covers terrible internet and thats why its not 0.0 seconds.

I made my own bot for it actually, had a support subreddit and everything, got banned from there for bullshit. Now you lot have to put up with me.