Heck, that's easy!
1. Just don't include any romance in the plot. Make it a "guy" book with all action/adventure, or an all "girl" plot with a band of FEMALE highwaymen.
2. Or have one of the people get "lucky", but not so much that there's any hope of continuation beyond a one-night stand.
3. Have the other person get killed after the one-night stand.
Unless, of course, are you telling an ACTUAL story from history that you're fictionalizing and there's REAL romance involved?
I guess I have to wonder one other thing -- is there a particular
problem with selling it as an historical romance? At 55% of the mass paperback market, that's not a terrible thing for your pocketbook...

I never planned to write romances, but it really is where the money is, advance-wise.