No, I'm busy

Hey!

Thanks for the email.

Unfortunately, I’m currently booked [until XX].

Best of luck! [If you’re looking for designers, I can recommend...]

Cheers

Potential reasons

You don't need to explain yourself!

I'm booked is more than enough.
But, if people keep contacting you for the same stuff, it sometimes helps to explain.

To be honest, this concept doesn't align with the work I'm currently looking to do.
Your idea is great, but unfortunately I can't make it work in the requested timeline.
The rates you mentioned are far below what I generally charge my clients.