ebooting the session host will obviously clear the issue but that can be difficult to arrange during business hours.

The alternative method is to log into Citrix Studio and launch Powershell by highlighting the Citrix Studio root level and clicking on the PowerShell tab as shown below.

You can then run Get-BrokerSession -username DOMAIN\affectedUsername | Set-BrokerSession -hidden $true to hide the user session in question and allow the user to disconnect the Citrix session and log in again without either picking up the same session or getting rejected due to reaching session limit.

Finally,  schedule a reboot of the session host for the next available opportunity.