How do you prevent a category from having entries posted to it

You can't stop the category from being used but you can prevent it from popping up as an option for new transactions that you enter: Tools > Category List > on the right side there will be a “Hide” column…check the box in that column for the Categories you no longer want to be used. After hiding the categories they should no longer pop up in the categories selection field when entering new transactions. If they still show up, make sure to uncheck the box for Show hidden categories when the popup showing the categories to select from is visible.

Note: Even though a category is hidden it can still be used by checking that box for Show hidden categories, again, or by simply manually typing in the category.