Yes, the ability to choose another LifeTime Wish disappears when your sim has fullfilled their original one. Stupid if you ask me. Again, "What were they thinking!" In Sims 2 at least another one rolled up or could be changed via the Orb. You could pursue it or not, as you pleased. It should be the same for Sims 3, leave the ability to choose it in the menu, if you don't want to purchase it, don't. If you want your sim to have a new goal, you can.
I am an Angel who has tamed the Dragon. For I am NOT crunchy, NOR good with ketchup!
In order to change a complete lifetime wish you'll need at least the base MC mod and the cheats expanded module. When they're both in you're game, click the chosen Sim, then NRaas -> Intermediate -> Lifetime Wish and choose what new Lifetime wish you want them to have.