Traps, I had the same problem with some warrants with a full brokerage, so don't think the online brokerages are any different. Also the brokerage I deal with CIBC Wood Gundy won't short my warrants because I have shares in my account. They don't care about generating a taxable profit because of my low purchase price average.

I could talk about taxes in a little more detail, but it might spark too much discussion.