SES Verified Sender Emails marked as Spam

Recipients can only mark an email as Spam on their side, they cannot mark the verified sender.
This process of amrking an email as SPam is called an abuse complaint:
https://aws.amazon.com/blogs/ses/email-definitions-complaint-rate/

When a recipient marks an email as Spam, these will cause the ESP (if supported) to send an ARF (Abuse Reporting Format) email back to AWS:
https://en.wikipedia.org/wiki/Abuse_Reporting_Format

Not all ESP support complaints (FBL - Feed Back Loop):
https://aws.amazon.com/blogs/ses/email-definitions-feedback-loops/

The chance of only one customer marking an email as Spam and affect the customer sending is very low, since it will depend on the number of sendings and if the ESP supports FBL with AWS.

AWS recommends that the complaint rate should be below 0.1%, over 0.5% customer risk having their account suspended. This is explained at the end of the first link I sent.

AWS recommends customers to monitor complaints and bounce rate in order to identify and correct issues as soon as they appear:
https://docs.aws.amazon.com/ses/latest/DeveloperGuide/monitor-sending-activity.html

Hope this can clarify you.