IMO, people won't go somewhere that looks inhabited (i.e. they are likely to be spotted straight away). If a bum walks past a two acre lot with broken cars around and it looks not looked after, they are going to be more likely to venture inside as the likelihood of them being seen is minimal (or so they figure). A dog may cause some hesitation, as well as barbed wire/black berries. However, if I saw a place that had barbed wire around it, I would assume straight away that they had something worth stealing.
Clean up the yard...get rid of the broken cars or at least move them to the side. Plant lots of plants (not cannabis plants I mean). Flowers are great and gives the casual observer the impression that you are taking care of your place. Look after them. Build a big veggie patch. Corn, climbing beans and zucchini are great companion plants. Corn eats nitrogen, beans fixate nitrogen and zucchini covers the ground. Beans climb up the corn and the corn MAKES A GREEN FENCE for you to plant some cannabis plants behind.
Dig 4 holes a few metres apart and put some big stakes into them (about 2 metres tall). Wrap some shade cloth or something around that and grow your weed in that. If you are growing other plants you will have lots of pots of flowers etc around and peoples attention will be on all of the different plants flowering and looking pretty. Their attention will be drawn to your veggie patch as well. The shade area will be assumed to be a seedling nursery for your veggie patch, or at least you can tell others that. Plant some rosemary, thyme and lavender around any entrances, walkways on the border of the property, it works well if their legs rub on the plants when they walk past so that the plants emit their smell (hopefully covering up any marijuana smells that might waft out). You can make those from cuttings from other peoples plants to save you time from growing from seed.
If you are still concerned about people wandering in, plant some stinging nettle in strategic places so people would have to walk through them to get to your cannabis plants.
I know this sounds like a lot of work, but it won't just keep bums out...the police are less likely to assume that some bum is growing marijuana on their 2 acre property if you take care of the place as well.
Hope that helps you some.
Cheers