This happened three years ago?

From what you wrote it's not clear exactly what happened. Did you pay the accountant to prepare your 2015 tax return, but he followed fraudulent instructions from someone else to change the account for deposit of the refund, and didn't contact you to confirm the change? If that's what happened, it should be the accountant's problem. See if the accountant will make good on it. If not, consult a lawyer, and file a preparer complaint with the IRS and with your state accountancy board.

If that's not what happened, please explain in more detail.

See the following link on the IRS web site for information about filing a complaint about a tax preparer.

<a rel="nofollow" target="_blank" href="https://www.irs.gov/tax-professionals/make-a-complaint-about-a-tax-return-preparer">https://www.irs....>

Yes, 2015 taxes filed in 2016.  I took it to the taxpayer's advocate and in 2018 they responded that the refund was deposited as they were instructed by my accountant to the imposter's account and they did not have authority to get the money back from the bank.  They said it was a civil matter.

Did you file a police report for theft?

I did file a police report and a complaint with FDIC.  The police tried to track down the email address of the imposter.  Nothing came of either.

The imposter phished email communication between my accountant and I, and redirected email from my accountant to him.  The accountant had no idea that my email address was taken over and I didn't know either.  So when the imposter told my accountant to desposit the refund into his accounts (overuling my original instructions to send my the check as had been my instructions in previous years), the accountant thought it was me.

Why?  The accountant is not at fault.

I feel that the accountant is ultimately at fault. Email instructions to change the account for direct deposit should have been a red flag, or at least aroused some suspicion. The accountant should have used an alternative means of communication, such as a phone call, to confirm that the instructions were genuine.

I disagree.  Unless email communication was unusual, that definitely would not have raised any red flags for me.  If I get an email from a 'known' email address, I assume it is genuine unless there is something within the email that seems suspicious.  Many of my clients only communicate with me via email, and it is common for them to change their minds about things.

At any rate, before the tax return is filed, the taxpayer must receive the tax return (which includes the Direct Deposit account information) and sign the signature form.  Assuming that was done correctly, the taxpayer had the option to review the return for accuracy (including the Direct Deposit information).  If the taxpayer did not actually sign this tax return, then that is grounds to get this situation fixed.