I'm pretty sure you cannot write off RSX yet because the fund hasn't been terminated and you may continue to make payments. The person who replied to you before said that you can't write it off until you are 100% sure it is worthless; that doesn't seem to be the case here, as RSX still holds shares in companies, and you may still continue to receive additional distributions.
Also, here's info from google on box 3 in you 1099-Div: (it) Shows a return of capital. To the extent of your cost (or other basis) in the stock, the distribution reduces your basis and is not taxable. Any amount received in excess of your basis is taxable to you as capital gain.
You aren't getting taxed on anything you see in box 3. If you decide to write it off, you should reduce the basis by the $70, but I don't think you can write it off yet. I'm trying to figure out the same thing myself, and this is my best assessment, but I'm not positive.
