The debate is on in Portugal
about leaving
the euro zone.
It is not the euro as such that causes the problem, but rather the
deficit spending rules of the euro zone.
A country outside the euro zone could still use the euro as its
currency. Panama and Ecuador use the US dollar as their currency, and
the US cannot stop them (and probably would not want to, given the
advantages to the US of the use of dollars in commerce elsewhere).
Likewise, Portugal, Greece or Spain could leave the euro zone and
continue using euros.
However, this would have disadvantages as well as advantages. some of
these countries may want inflation to reduce their debt, and if they
tried to inflate the euro they would arouse much bigger hostility.