Unfortunately there is no way to directly target countries by exclusion. The best way to avoid displaying a message to a specific country is to create a second bar targeting that country - perhaps with a generic welcome message. Since the app always displays the most targeted bar, visitors from the country you want to exclude will see that one instead.
How do avoid displaying a bar in a specific country
1. Create a bar targeting the country (or countries) you want to exclude
Create a bar with a generic welcome message and target it to the country you are trying to exclude. The app always picks the most targeted bar to display, so this bar will always display to visitors from this country.
2. Create an untargeted bar
This bar will contain the message you want to display to everyone else. Any country that is not specifically targeted by a bar will see this one (with the real message).