We have seen this on occasions when a request to stop the project is made from the portal but the system is processing a large item which it needs to complete before responding to the portal. Sometimes the portal times out waiting and leaves the system still showing as "stopping".
Please allow 15 minutes and then if the project is still showing as "Stopping" select the project tick box and click stop again.
If your project fails to stop there will be a different issue and we request that you contact support by raising a ticket.
This issue is being investigated further and modifications will be made to prevent this from happening in the near future.