Hello Sarah,

There are two main concerns with cats using garden beds as litter boxes.  The first is the possibility of Toxoplasmosis.  Toxoplasmosis is a parasite carried by cats that in pregnant women can lead to miscarriage or fetal malformation, Toxoplasmosis - Frequently Asked Questions.  You should discuss this disease with your physician. 

 The second concern is that cats like to use areas that smell like their waste, so it's important to remove all sources of that smell, otherwise they'll be back.  Cat urine and feces contaminate the soil, but can also leach into the frame of the raised bed.   To prevent use of your raised bed as a litterbox you'll probably have to remove the soil, and clean and/or replace the frame if it's wood.  Sorry.

  Cats like to dig in loose soil, and to cover their feces.  Unfortunately, your nicely turned garden soil is perfect!  There are a number of ways to discourage:  this lay chicken wire laid on top of the soil;  use bird netting over the bed and plants; motion-detector activated sprinklers, even plastic forks poked into the soil tine side up.  Additionally you can create a more attractive alternative - an area with litter, sand, or fresh dirt where they can urinate and defecate freely.

Always use gloves when cleaning your bed.   Fresh soil, protected from cats will be best.  This article, Keeping Cats out of Gardens and Yards has additional ideas.