Is it a crime if you have a power of attorney and as agent use the principal's social security number?

Yes, it is a crime to use someone else's social security number to obtain a credit card and charge it up.  It is not only identity theft but it a breach of fiduciary duty and it a degree of theft.  What do you mean who charges the individual?  You need to report it to the police and then the prosecutor would be the one to levy the charges as against the individual.  In the meantime, you need to revoke the Power of Attorney.  You should contact all the banks at which there may be an account and advise them that this person no longer has an rights to represent you as an attorney in fact.  If there are any other places that you can think of that may extend credit to the person, contact them as well.  Good luck.