I would say: 'No thank you', or 'No thanks'.

Thank you!

It is a common question. The easy way to say "no" it is "sorry i'm busy, cann't go with you". In Russia if we want to say "no" to someone, we just say what we don't want to go out with smbd, and you are not braking your relashionship. I hope what i helped you. c: Also you can say " Net, ne hochu!", it means " no, i don't want smth!"

Thank you for telling me :)