How about, instead of saying No, try:
"You seem like you could use another set of tutoring. How does $XXX (insert mad benjamins here) sound for 2 hours a week?"
Then it's up to them to say yes or no. :P
Another thing you could practice is, "No thanks, but thanks for asking!" (with a sincere smile) even if it doesn't seem a 100% fit with the question asked. Then stop talking...don't justify, don't give in, just...don't. With a smile.
Wash, rinse, repeat as needed.
No, I can't tell you how to say no, but thanks for asking! *bright smile*