Boy am I feeling old. You are right of course that SEO does it all by itself now. Funny that, even though I thought of using SEO, I still fell back to the old save as object trick. I think I need more sleep.
SEO = Solid Element Operations. Look it up in the manual or help files. Please don't let a lack of understanding turn into a diatribe. We all know that computers are supremely frustrating when we don't know how to get them to do what we want, but a softer tone is more likely to elicit a helpful response.
Link has already identified the "free line cut" method you ask for - as close as it can be in ArchiCAD:
to use the Section line tool in 3D via the special menu - but this doesn't allow vertical definitions like the SEO knockout method we have just discussed - ooops. it does....
You can also define your visualization area with a complex marquee, selecting only the storey you want to visualize.... and preserving the marquee path by using an outline on a special layer....
each of these are only one or two steps...... but feel free to add your request to the official wishlist. If you were to upgrade to Version 8.1, I am certain that you would find these alternatives adequate.