Run the vxdiskadm program and select Remove a disk from the main menu, and respond to the prompts as shown in this example to remove mydg02:
Enter disk name [<disk>,list,q,?] mydg02
VxVM NOTICE V-5-2-284 Requested operation is to remove disk
mydg02 from group mydg.
Continue with operation? [y,n,q,?] (default: y) y
VxVM INFO V-5-2-268 Removal of disk mydg02 is complete.
Clobber disk headers? [y,n,q,?] (default: n) y
Enter y to remove the disk completely from VxVM control. If you do not want to remove the disk completely from VxVM control, enter n.