Is this an image, or out-of-band signal?
If the former, carefully reconsider your band plan: place the VFO as far from the RF band and harmonics as possible. The harmonics of both LO and RF are relevant, to avoid higher order mixing products. This works when the received band is relatively narrow, for example you can receive FM BCB (88-108MHz, a 20MHz span) with a ~30MHz IF (LO 118-138MHz) which has an image at 148-168MHz (easily filtered) as well as good distance between 2nd and 3rd harmonics (giving good overload rejection).
Note that a traditional AM BCB receiver accomplishes this by actively tuning the front end: it's tunable over the full BCB, but the bandwidth actually received at any given instant is a small fraction, and this provides image rejection. (With a 455kHz IF and a tunable range 530-1600kHz, the VFO range overlaps some of the overall RF band!)
Traditional FM BCB receivers can do this as well, allowing a 10.7MHz IF, slightly more than half the RF BW and just barely avoiding in-band images.
If the latter, just improve the filters..? Maybe you need some shielding to help them out?
Tim