Ever been barred in a country

Barred? What are you talking about?

When the country doesn´t allow you to enter because you´re American, or form a Third World Country, whatever reason

Wow, I've never heard of that.

Hm, well I know of cases where a Moldovan citizen has committed a serious crime or a series of repeated crimes in Italy. After his prison time in Italy he was barred from ever reentering the country. A year later or so he happened to bump into a regular police control in the German-Dutch border region and they found out he was trafficking drugs from Eastern Europe to the Netherlands. German plolice then found out about his past in Italy but since that had nothing to do with the current situation and since he wasn't even touching Italian soil he only got whatever penality you get for trafficking that amount of drugs...You know when you're barred from a certain country as it clearly says it in your passport. So there won't be any bad surprises. To the best of my knowledge neither US nor EU citizens are barred in general from any one country in the world.

I know US citizens need a visa to travel to Russia (and probably to Belarus), but I know of no other such restrictions in Europe. Presumabley, if a traveler has a significant criminal history, they may not be elligible for a passport in the first place.

We know of no country, with possible exception of North Korea, that "bars" entry to people solely bcuz they are Americans. Some countries require visas and if you do not have one, you cannot enter the country, The US has recently begun requiring visas anf that has greatly reduced travel for vacations to this country.
We personally feel that visa requirements are silly and a pain, so we avoid nations which require them, like Moldova,Russia and Belarus.

I agree, you would need to do something to prompt the action, either criminal in that country, or related to visa/immigration issues. I have heard of people having visa privledges revoked and in essence barred due to overstaying a visa (even the 90 day tourist limit that requires no formal visa) The original poster lists Amparo as their location, I assume Brazil, so many more visa and entry requirements than we in the US enjoy.

As an example of visa penalties, upon exit they have the ability to hold you, can fine you ($100's of dollars) and can effectively prevent re-entry into the Schengen countries for up to 10 years. Still, routinely on boards such as this there will be questions regarding "chances" of getting caught, how to get around a visa, etc.