Ann: Market Update - Congo and Cameroon, page-205

You can't claim a loss until the company is wound up or sold. It's not enough to be de-listed, you still own a share of the company right now, it's just no longer publically tradeable. It is possible to trade unlisted shares but I've not gone through it before.
I had this happen with CCC Continental Coal and it took a couple years for administrators to finally take control and wind the company up, they then provided me a letter saying that at this date the shares are deemed worthless and can now be shown as a loss.