Honestly, your question could be (and probably should be) shortened to:
I'm really good at subject X, and students keep coming to me for help on subject X. How should I respond, given that I don't have time for this?
I'd advise the following response:
"I really wish I could help, but I don't have time. You really should ask your teacher, or teacher's assistant if you have one."
And you're done. There is no need to be concerned about how they came to know of your skills, or become involved in petty politics or whether someone is getting a free ride or passing the buck.
If the student indicates that they've already tried those resources, you might remind them that they are paying students, and if their teacher or TA aren't meeting their needs, they may have to have a discussion with them, or bring it up with their guidance counselors. Also suggest that they might want to form study groups with others in their class.
Lastly, if you enjoy teaching them, offer tutoring services. This will give you the ability to receive pay for your work, without all the overhead the TA has to deal with, and you're already receiving free advertising.
Regardless, there is no need to carry bitterness and acrimony into these situations. Treat it as though you hadn't been hurt, and as though the TA and teacher are both acting in the student's best interests, and then decide how to act from that point. The answer should be obvious once you ignore your personal baggage.