How do I clear any zpools before installation?

In a terminal type "man zpool" to review all the commands.
Not sure what you mean by clear....
You can destroy a pool should you wish and lose the data.
Or, if you wish to keep the data on the disk and remove the disks the data is on from the system you can export the pools.
Later you can import the pools.
If installing and you just wish to overwrite the data then do a full disk installation.
Steve