Man that really sucks. That spot is compromised and you are going to have to find a new one. I would make it a point to tell your friend that know you grow that because of getting robbed last year you are retiring. Do it often and loudly. Whoever did it is still probably going to check that patch out again this year, but if they know you are growing again they might look harder. I had my best patch that I have been using for 4 years now get found last year and had some plants stolen. Luckily, I had all the big plants out by then so my losses were minimal. The worst part is not being able to use that spot again. I did my first outdoor growing when I was a teenager and every single time I showed someone a plant it was gone the very next day.