I want to stand outside a shop with a banner to protest but do not know how to word it without it being slander. For example Don't BUY FROM THIS COMPANY

Just don't be standing on their property when you do this or you will be arrested for trespassing.

Waving a banner that asks people to boycott a store isn't slander. Calling the owner's mother a whore would be.

Judy's right, and you'd get more attention standing on the street anyway.