Oh yes they can!
CosmosSchwartz/Animalclub from the P&O Ferries website, under Ts&Cs:
"All tickets of Restricted Duration are only valid where the outward and return journeys are completed within the specified period of time, with the same vehicle and passengers and using the same operator. If you fail to travel on one part of your booking then we may charge you a supplement. That supplement will be the difference between the amount you have paid and the price for the standard single fare appropriate to the sailing used. This payment will be collected using the credit card details provided when paying for the original booking."
If you sign up with them then you sign up to their Ts&Cs!!