Canada

They could position themselves to not be as attractive, which i suspect goes against their current policy. The biggest issue is global confidence. I'm not sure they're there now, and with some of the issues with some countries' economies having a negative impact on the Euro, they may not be able to increase confidence sufficiently without hurting those countries or perhaps other countries to compensate for that. So not so much stopping others from using it, but not doing enough to raise that confidence sufficiently.

This is entirely speculation on my part, and not something I've studied intensely. I'm sure some economist could point out enough holes to make that look like a screen door, but there are only a handful of currencies that have the value, economic stability, and management philosophy to really fit the bill.