Once I sold a reseller hosting from https://softitcarehosting.com/reseller/details to a client. But some days later I found out that there was a relay alert coming from the reseller provider. Found out that the reseller client was providing support to a spamming website. I did suspend the reseller hosting and also reduced per hour deferred mail number per hour. Is there any security plugin that I can use to stop spammers from spamming from reseller services?