You can file a total loss of value or control of coins by going to Dashboard > Import More Transactions > Manual Entry > Outgoing, "Loss" from Dropdown.
For tax purposes, make sure you keep good records of the loss of your coins.
You can find how the IRS defines Casualties, Disasters, and Thefts here:
https://www.irs.gov/forms-pubs/about-publication-547
A casualty is the damage, destruction, or loss of property resulting from an identifiable event that is sudden, unexpected, or unusual.
· A sudden event is one that is swift, not gradual or progressive.
· An unexpected event is one that is ordinarily unanticipated and unintended.
· An unusual event is one that isn’t a day-to-day occurrence and that isn’t typical of the activity in which you were engaged.
Generally, casualty losses are deductible during the taxable year that the loss occurred.
Here is a video showing how to mark as Stolen/Lost: