You can use asmtoolg (GUI version) to delete disk stamps.
In directory where you unzipped the Oracle Grid Infrastructure image files, go the Grid_home\bin folder and double-click asmtoolg.
If user access control (UAC) is enabled, then you must create a desktop shortcut to a command window. Open the command window using the Run as Administrator, right-click the context menu, and launch asmtoolg.
Select the Delete labels option, then click Next.
The delete option is only available if disks exist with stamps. The delete screen shows all stamped Oracle ASM disks.
On the Delete Stamps screen, select the disks to unstamp.