Once you have tripped the romance line - and I think Nadia and, I've heard, Jorgan have stealth non-[Flirt] triggers - you cannot leave. There's an old petition
to let us, you know, break up with our companion characters and maybe not be bombarded with kisses, propositions, proposals and so on forever more, but as the game stands, once you've started one of the full romances (i.e. not Tharan or Pierce) you don't get to stop.
You might have just stumbled on a bug.
I had something similar happen with my (female) Consular. I had had normal conversations with Tharan, completely avoiding any flirtatious remarks, and then suddenly he wants to end our "liaison" as if we'd been in a romance.
In Tharan's case, that conversation opener is, I believe, a bug. Fortunately once he's finished his hilarious explanation of why "it has to end," he doesn't bring it up again, regardless of whether you encourage him or don't. He isn't a full romance, just an optional one-night stand with, it seems, a non-optional followup mention.