An offside goal which was allowed to stand had Barwell ahead early on and Jamie Towers added a second to give Marcus Law's side a 2-0 lead.
A penalty made it 3-0 before Kelvin Phillips pulled one back from a free kick after a Barwell player had been red-carded.
Before the end, another former Sphinx player in Jerome Murdock added a fourth for Barwell who were presented with the Joe McGorian Trophy.