Growing problem
While the practice of flooding forums with fake comments is most widespread in China, where such posters are called the Internet Water Army, it is becoming common in other nations too.
The US military is known to use fakes to infiltrate chat forums to gather information about potential terror groups.
Similarly many Facebook pages are plagued by bogus friends and "social bots" that are used to stage debates.
Many marketing firms also seed forums with comments in a bid to create "viral" interest in a company or event.
However, fakes can be spotted by analysing their patterns of activity and the words they use, say the researchers.
Fakes are more likely to start new comment threads, make inane comments rather than add to a debate, and repeat former comments with minor changes, the study suggests.
The researchers say they are refining software tools to help website administrators tackle the "painful" problem.
BBC News - Fake forum comments are 'eroding' trust in the web