I finally found the place where to manage issued vouchers. It is under tab Sales/Voucher in Store admin page and the tab only available when you access under administrator role. In case of the site which supports multiple sellers (Market place), the sellers can not revoke the voucher issued by themselves because the tab Sales/Voucher is unavailable in My Sellers page.

This may be a bug.

Cheers,

Tim

Sellers cannot issue vouchers currently for security reasons. Vouchers are like $ money so it's best left to the store merchant.