All credit for this one goes to http://www.tech-recipes.com/rx/2787/solaris_delete_zfs_slices_from_a_disk, i just wanted to post it here so i can find it easy next time.

amstel:/ # format -e c4t4d0
selecting c4t4d0
[disk formatted]

FORMAT MENU:
disk - select a disk
type - select (define) a disk type
partition - select (define) a partition table
current - describe the current disk
format - format and analyze the disk
repair - repair a defective sector
label - write label to the disk
analyze - surface analysis
defect - defect list management
backup - search for backup labels
verify - read and display labels
inquiry - show vendor, product and revision
scsi - independent SCSI mode selects
cache - enable, disable or query SCSI disk cache
volname - set 8-character volume name
!<cmd> - execute <cmd>, then return
quit
format> l
[0] SMI Label
[1] EFI Label
Specify Label type[1]: 0
Warning: This disk has an EFI label. Changing to SMI label will erase all
current partitions.
Continue? y
Auto configuration via format.dat[no]? y

Now i can use prtvtoc to lay out an empty partition table to the disks.

amstel:/ # prtvtoc /dev/dsk/c4t5d0s2 | fmthard -s - /dev/rdsk/c4t2d0s2
fmthard:  New volume table of contents now in place.