Matzilla, unfortunately, there is no chance of removing all the bullheads by seining.
I would strongly suggest a draining, sterilization with Hydrated lime, and restocking. It would not be difficult to lower the pond as low as possible and selectively save several fish to restart with, but needs to be well planned for sizes and numbers kept.
I recently drained, sterilized, removed/rehomed dozens of large Koi and restored a church pond in Tulsa for under $2K. Seining a fishing pond is usually extremely difficult, if not impossible due to the usual structure present, not to mention a foot to feet of sludge, muck, and soft clay in the lowest parts of the basin.
