If you have a non-performing Investment, which has no longer any value, you can close it with a valuation equal to zero. In the overview page of the Investment find the three dots next to the name of the Holding, and select "Close": you will be able to record the write-off date and the set the value to zero. 
The Investment will no longer be visible on the left menu, but you can access it in the Holdings table on your Home page, in the "Closed Investments" section. The metrics on your Home page will include the (negative) performance realized on the Investment.

If circumstances change and you believe that a recovery value is possible, you can always reopen it at a later stage.