안녕하세여.
I appreciate the offer, but i have to decline.
Thank you for thinking of me, but I must say no.
I regretfully have to pass on that.
I'm sorrym but i have other commitments.
It's just my policy.
I won't do anything that you ask me to do.
I'm not prepared to wait any longer.
등의 표현을 활용하시면 좋을 듯 합니다.