Unfortunately, you can't "simply write it off" for tax purposes....

Unfortunately, you can't "simply write it off" for tax purposes. You need to either sell the shares, or obtain written evidence that the shares no longer exist. You won't get the second because the shares do continue to exist, so you will have to pursue the other option. If it were me, I would reach out to the company secretary to discuss the options and procedure for selling your interest. If the company won't buy them back, you could try selling them to a third party like delisted.com.