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.
What he said.
4 times in the last 2 years, I've seined, drained, reseined, and then completely drained my 1/4 acre pond to the muck, and every time there are dead fish from the hydrated lime. It takes only a small gap anywhere the net touched the bottom to allow fish to escape. Even a fallen limb on the bottom of the pond will create a gap that fish can escape by.
