You speak in contradictions. For example, a 1946-1964 Roosevelt dime has a face value of $.10 but a "melt value" of $1.2022 (today) according to http://www.coinflation.com/
The U.S. government may not like it but there is little they can do about someone melting down coinage, especially outside the country. (But don't do it -- silver coins are worth more than their melt value).
Speaking as a regular person and not a numismatic geek, I don't know anyone who will give me melt value for my coins, and I wouldn't think it was worth the trouble of sorting out the old ones anyway. Thus all pennies are worth one cent to me and it isn't possible to "debase" them.
The U.S. government may not like it but there is little they can do about someone melting down coinage, especially outside the country. (But don't do it -- silver coins are worth more than their melt value).
Paper money is different, of course.