If you are low on disk space, the first step would be to delete the snapshots.
Anyway, the reason why the GUI doesn't allow you to delete the virtual disk is, because this would result in an invalid snapshot, i.e. make it impossible to revert to the snapshot. You can certainly edit the configuration (.vmx) file manually and then do some cleanup (i.e. cleanup the .vmsd file, delete the ,vmsn file and the old/cloned .vmdk files). After editing any of the files make sure you reload the VM's configuration (see http://kb.vmware.com/kb/1026043)
André