Hi,
yes, there are "one-way cruises". These are mostly repositioning cruises including trans-Canal, trans-Atlantic or coastal cruises to name just a few. Also, some cruise lines do offer the opportunity to stay in one of the ports of call visited for a week and then resume the cruise the following week. A good cruise-specialized travel agent should be able to help!
Best regards,
Raoul Fiebig
Das Ruderhaus