Whoa, whoa, whoa, hold up a sec: Pit bulls, are NOT human aggressive. Well bred, well owned, and properly trained/handled American Pit Bull Terriers are one of the LEAST human aggressive dogs around, and they make TERRIBLE guard/personal protection dogs. Period.
Secondly, DOGS ARE NOT SECURITY SYSTEMS. The BC is a high strung breed as it, training it to "scare people away" on command, etc... is a law suit waiting to happen. They also were NOT bred to be guard dogs, they are HERDING dogs, plain and simple. Unless you plan to spend the thousands it can take for professional personal protection classes, you're going to create a time bomb more or less. You need protection, get a can of pepper spray and a tazor and call ADT. Don't pass your insecurities off onto a dog by training to "scare people away" on command, which is an attack waiting to happen.
ETA: Wanted to add this, as the first person said, you'll also need the RIGHT dog. If the dog, the genetics behind the dog, etc... don't have guarding in them, they will NOT make a good guard dog, and even training them this way, will do little to no good. Genetics play a HUGE roll on this. Even some of the BEST guarding breeds have individual dogs that are horrible guard/protection dogs. So unless you plan to find a dog bred specifically for protection work, and then still end up paying the thousands of dollars it costs for a professional personal protection class, you're asking for something to go horribly wrong.
ETA 2:
1.) A dog can be shot, and then you're screwed
2.) When you go practice soccer? So then that? Leave the dog tied to a random tree? And what happens if some random person comes by, while you're practicing, sees the dog, and thinks it'd be a great idea to take it home?
3.) Again, dogs are NOT security systems! Training them this way can very well make them leery of any and ALL strangers, and if they aren't controlled well enough, they could very well take matters into their own hands and become aggressive towards anyone and anything that's not you. Creating a protection dog, in a DIY manner, (especially with a dog that wasn't even remotely BRED for this type of work) is basically going to lead to the equivalent of slapping yourself in the face with a handful of Napalm.