You know, I really do whatever I can to prevent line jumping. At Space, if someone claims that their party is at the load area we tell them too bad. Now, if they are at greeter and tell us their party is in line, we will generally allow them to rejoin simply because we know that they can't really get past our merge point and usually see them coming because nobody ever knows where the restrooms are.
Line jumping can be very difficult to prevent though when we have a full concourse and things. Either way, if I have more than one witness, the guest is out of the line and I generally walk them back to the beginning. I have only had one instance where I had to threaten to call security. I pulled out my cell phone and the problem was instantly solved. (And before you yell at me for pulling out my phone, the guest was getting confrontational and I had no other access to a phone at the time.)