Condor Ferries has recently faced a series of problems with its new ship, the Condor Liberation, and said staff had been verbally or physically abused.
The sign reads "if you're abusive, threatening or disruptive...you won't be travelling with us."
It has been placed in ports, aboard the ships and in other places staff are in contact with customers.
A spokesman for Condor said: "This is following a series of unpleasant events where customers have threatened or been abusive to staff, either verbally or physically.
"Our hardworking staff do not deserve to be subject to this kind of behaviour.
"We are also distressed to read some of the comments about individual employees which have been circulating on social media and would urge the public to refrain from this sort of intimidation."
The Â£50m vessel's first problem came after suffering minor damage while mooring in bad weather in St Peter Port, Guernsey, the day after its first commercial sailing.
Since then it has had to cancel a number of sailings or be redirected due to problems with high winds.
The spokesman added: "Whilst we recognise that the reliability and punctuality of Condor Liberation has been below the standards which our customers and we expect, the latest data shows a steadily improving picture.
"Condor Liberation has now operated 92% of her permitted sailings since coming into service, two thirds of which have been early or on time."
Signs warning ferry passengers against abusive behaviour have been posted by a Channel Islands boat operator.