Excluding Items from Search?

Hi! I bought the plugin and it is working great It’s just my user error that needs attention. A lot of customers will want to do one of two things: 1) Filter recipes so that it shows recipes that “include” a certain ingredient (ex. beef). However, there are also a lot that have pets with allergies. They need to be able to select “exclude beef” and only recipes without beef show. How do I do this?

The page I need help with: [log in to see the link]