An inline filter does exist, typically a commercial product. However, it will not differentiate between digital or analog, it will block both equally well.
Your best hope is to use the most directional antenna available. Aimed at the desired station and perhaps more importantly, aimed so that it's least sensitive direction faces the station that is causing the interference. Your situation is one that calls for "walking the roof", meaning, moving the antenna up/down and laterally to find a "sweet spot" were the desired signal is strong and the interfering signal is weak.
A possible candidate is the 91XG
http://www.solidsignal.com/pview.asp...u=853748001910
Your TVF report indicates about 15 dB difference (in you favor) between the to two signals. That makes me think there is hope.
If you are very committed, consider:
http://www.kyes.com/antenna/stackluge.html ,
http://www.hdtvprimer.com/ANTENNAS/ganging.html ,
http://www.anarc.org/wtfda/stagger.pdf the intended result is a more directional array. Done right and with a bit of luck, stacking identical antennas can help in marginal situations.