Hi @Newtonian,
Welcome to the Discuss community, and many thanks for reaching out about this! 
We want to mention that we are not able to provide tax advice, so please do check with your accountant/CPA to see if this treatment is best for you.
The options to do this in Koinly would be creating a manual Lost (withdrawal) transaction and set the cost basis of those transactions as zero to realize the loss. Alternatively, the transaction can be tagged as lost which nullifies any gains.
Here is what the pathway of creating a manual Lost transaction would look like in Koinly:
- Add Transaction > Withdrawal

- Select Lost access / Stolen from the dropdown menu

- Fill in the date/wallet/number of tokens. The date can be whatever you think is appropriate, i.e. when Celsius stopped withdrawals or even just today’s date.

- Save and Koinly will then recalculate,
The original cost-basis will then be added to the Lost section of your tax report so you can report this when you file.
Please let us know if you have any issues in the process. Our support team is keen to help at any step!
Thank you again, Newtonian. Have an awesome day!