You need to replace the joker valve. On most heads it is located where the waste line meets the bowl. Between the bowl and the hose you will find the joker. Remove it and replace. The new joker will [likely] include an installation slip providing specific directions.
This is an example for Raritan: RARITAN JOKER VALVE Shop.Sailnet.com - sailing resources, shopping, sail, blogs
Here is a Jabsco example: Joker / Check Valve Kit*
[apologies Sailnet: I couldn't find a Jabsco joker in your store]
Start by flushing as much water as you can to clear the line. In any case, have a large bucket at the ready - wear rubber gloves. The waste in the hose will discharge in your direction. Depending on your head manufacturer, removing screws or hose clamps is all of the mechanical effort. The rest is just... well obnoxious!