Prerequisites: The Microsoft.RDInfra.RDPowerShell module, the Az PS module
First, install the RDInfra module:
Install-Module -Name Microsoft.RDInfra.RDPowerShell; Import-Module -Name Microsoft.RDInfra.RDPowerShell
Then proceed by installing the Az module and logging in:
Connect-AzAccount

Once you are logged in you can run the following script to disconnect a specific user session:
Get-RdsUserSession -TenantName "tenantname.onmicrosoft.com" -HostPoolName "HostPoolName" | where { $_.UserPrincipalName -eq "azvise\demouser" } | Invoke-RdsUserSessionLogoff -NoUserPrompt