My friend is in web hosting business accepting Bitcoin. There is an alarming level of customers using bitcoin spamming emails out from his servers. Due to the anonymous nature of BTC users, abuser signed in as another customers and continue the spam activities. Multiple servers have been taken down by network providers due to spamming activities.  

I have been helping him to look for different ways to detect such users up-front during checkout. We are using FraudLabs Pro and it detects highly anonymous, blacklists and recurring abusers.

Are there other screening services we can use? Preferably with free plan like FraudLabs Pro.

Reference: http://www.fraudlabspro.com/tutorials/how-to-prevent-bitcoin-fraud