It's still hard to say no (though the pandemic made it a whole lot easier).
ALSO, WHO ASKS IF YOU'RE BUSY? That seems rather presumptuous, but then again, Pam has already demonstrated that she has zero respect for the two of you.
Tell her to shove off. Or that you have to trim Charlie's ball hair. Or shave his back. Or that you just don't want to because she's disrespectful.