I'm not sure of this, but, if you start romancing someone and you are already in a relationship, don't you get an option to "break-up with x"? I have never had my sim in a similar position so, I am not sure.
I take it that your romantic interest comes from one of the WA locations. I once had one of my sims marry a WA character and have a child without even knowing about it. It was most inconvienient as I had her lined up for marriage to a carefully chosen local sim. I knew which town the unexpected husband lived in so, I went to edit town in that location and evicted him (and sadly the child too). I left them on the clipboard when I saved the town and returned to my game. Every vistage of them was erased. Of course, they need to be a housed sim, not a homeless sim, to do this.
I see that you say the tourist is no longer in the relationship panel. So, have you tried just starting a new relationship? Isn't the scrapbook (which I keep turned off) just memories?
If this problem is just a glitch, then, resetsim xxxx xxxx may fix it, or alternatively, using edit town to evict your sim family then immediately replacing them in their current home often fixes glitches. But, I would think that whatever you do, the memory will remain in the scrapbook until, if you so wish, you manually delete the memory.