You can exclude keywords by preceding words or phrase with a minus sign, e.g. "spokeshave wood -brass", and you can also omit phrases---if you put the phrase in quotes. (-"remove this")
For countries, a "Region" pull-down would be programmatically easier. In that case, choices are North America, Europe, Asia, Oceanic, South America, Africa. Supporting countries would be tough because there are currently 190, with more being added each year.
For categories, we can put in a category pull-down of major categories for you. I also considered a number field to just search a very specific category---would that helpful?
So please let me know about
1. Regions instead of categories
2. Category number field
3. Any other ideas
bidslammer