How do I mitigate my group project members plagiarizing off me and taking advantage of me?
May I suggest an alternate point of view? Is it possible that you are grabbing the reins of the project and trying to control it too tightly? 

I bring this up because you mention a couple of times that you are the only one who knows how to program. From my own experience I know that when experienced programmers are compelled to work with inexperienced programmers there is a strong temptation to seize control of the project in order to make sure it is done well. You have to keep in mind though that this is an educational exercise. The goal is not the completion of the project per se, but that the students learn the material by participating in the project. You may be assuring a higher quality result by taking control of the project, but you may be interfering with your fellow students learning the material. It may be time for them to learn how to program.

This may be doubly difficult because it sounds like you are more motivated than your fellow students. I expect that you knew exactly what needed to be done and wanted to come to grips with it, while your fellow students may have been "Yeah sure, whatever". Consider this an opportunity to learn how to engage and motivate your co-workers. You are going to be working on "group projects" most of the rest of your career, so this is an important skill to cultivate.

Of course on top of this is your natural concern about the project grade. I don't know how to ensure that your desire for a top grade doesn't conflict with the needs of your fellow students to get hands on experience or vice versa. I can only suggest that in the worst cases where one of your fellows is simply sitting back with folded arms, you may need to discuss the situation with the professor.