As @TimRice pointed out:
The SOAP API is “read-only” since it actually work with the Web Report database.
I believe it would need to make use of the REST API to be able to “Mark a Computer as Deleted”.
Currently, the module just speaks the SOAP API. REST API support is coming, but there are a few additional capabilities that have to be developed first (like the ability to treat content [actions/analyses/fixlets/tasks] as first-class objects vs .BES XML) to enable that.
You can however leverage the direct PowerShell commands do accomplish this if you want.
$Credential = Get-Credential
$ComputerId = "COMPUTER_ID_TO_DELETE"
Invoke-RestMethod -Uri "https://bigfix:52311/api/computer/$($ComputerId)" -Method Delete -Credential $Credential
If you leveraged New-WebReportsSession previously (say to query for the computer id to delete) and your Web Reports credentials are the same as your Console Operator account, you can replace the first line above with:
$Credential = (Get-WebReportsSession).Credential
Just keep in mind that error handling and the processing of the Invoke-RestMethod call are not shown and you should handle this.