Actually, that is not technically true.
'What is 'Bankruptcy'
A legal proceeding involving a person or business that is unable to repay outstanding debts. The bankruptcy process begins with a petition filed by the debtor (most common) or on behalf of creditors (less common). All of the debtor's assets are measured and evaluated, whereupon the assets are used to repay a portion of outstanding debt. Upon the successful completion of bankruptcy proceedings, the debtor is relieved of the debt obligations incurred prior to filing for bankruptcy.'
Read more: Bankruptcy Definition | Investopedia
Bankruptcy Definition | Investopedia
As you can see, bankruptcy can be instigated by creditors. So, if Venezuela gets so far into debt and it's currency becomes practically worthless OR others refuse to accept it as currency, then Venezuela could be forced into bankruptcy by it's creditors.
It is not likely to happen...but it is technically possible (to my knowledge).
The thing to remember about currency is it is only as valuable as others think it is. If all others think it is worthless...it IS worthless.