First of all delete all disks from diskgroup

/usr/sbin/oracleasm deletedisk DISK1

then
 clear asm disk headers
dd if=/dev/zero of=/dev/mapper/mpath0p1 bs=8192 count=12800

reconfigure asm disks
 /usr/sbin/oracleasm configure -i

and scandisk again
/usr/sbin/oracleasm scandisks

recreate asm disks
/usr/sbin/oracleasm createdisk DISK1 /dev/mapper/mpath0p1

last thing , you have to reconfigure asm disks on other node

/usr/sbin/oracleasm configure -i

thats it..