When racist graffiti appeared near Thomas Turnham's flat in Herne Hill, false accusations began to appear on Facebook and Twitter. People swore at him and called him a "racist pig".
But Thomas wasn't just innocent of the crime - he was 300 miles away in on holiday in the Lake District. Checking his e-mails, he found hundreds of abusive comments, and the messages kept on getting worse.
The graffiti - which was scrawled outside a bar - has now been removed, an police have confirmed that Thomas is not suspected of doing it.
But the teacher, who works at Dulwich Hamlet Junior School, is worried the allegations will hang over him. The police say they can't take action against his online abusers, because it's a civil matter, not a criminal one.
Thomas wants to see a change in the law to protect people from online abuse. He spoke to our reporter, Ruth Zorko.