FAke registrations and visitors

Hello to everyone in this great community...

So here's what has been happening.

I've received notifications of several of my sites exceeding my bandwidth, I went to google analytics and noticed that I have thousands of visitors from Russia, Ukraine and other places and countries. I have also seen in the past few days a lot of registrations to my clients sites....

Any advice on what to do?

All tips, tricks, hacks, anything you recommend...

I found some security plugins such as sucuri, wordfence, BulletProof Security with good reviews but I don't know which way to go...

Thanks for your help