Not a user of Avast, but being charged for it.

[jjohnson00]

I am being billed for Avast and I do not have it on my computer. How do I get a refund for all charges?

[DavidR]

You don't have to have it on your computer as such.  If you previously had it and had a payment mandate set up it would renew.

You would have needed to cancel the Auto Subscription renewal/payment/mandate.
-  Cancelling automatic renewal of the Avast subscription - See https://support.avast.com/en-en/article/35.

Refund request - https://support.avast.com/en-ww/contact/sales_refund If you haven't got the Order ID, etc enter 00000000 and give as much information that you can to help identify it, the email used, etc.

Also see -- Sales - Orders and Renewals FAQ - https://support.avast.com/en-ww/article/Alternative-unsubscribe/. 
Scroll down to the "How do I cancel automatic renewal of my Avast subscription?" option.