0. Shutdown the VM, and ensure it's powered off.
1.Right Click the VM, select Snapshot > Take Snapshot (no need to tick memory or quicese) Enter a name e.g. test
2. Wait 60 seconds at least.
3. Right Click the VM, select Snapshot > Snapshot Manager > select the top level snapshot > DELETE ALL

This will remove and merge ALL snapshots (consolidate/committ) into the parent disk....

BE PATIENT and WAIT....Check VSphere client tasks for progress.