To find out what stairs you can currently challenge someone in, first go to the Main Stairs Page, then click on "Show your stairs only" link. The ones in BOLD are ones you can make a challenge in. .
actually people who are inactive (either from the site or the stair) will take care of themselves over time, they will fall to the lowest step and the stair will grow well past them, the only real problem is the fact that many people don't like cheap wins