Bring in free bran muffins with chocolate chip topping and dried fruit inside.
The chocolate chips are the linch pin in this plan. They are often irresistible to the greedy hands of office zombies. The real treat is in the bran and dried fruit. Many will disregard the bran in their zeal for the chocolate chips. That, combined with the dried fruit, may cause lines for the washroom.
Examples of fruit that produce good results are raisins, prunes and dried apricots. For best results you may want to chop and mix a variety of dried fruits for your concoction. For a more chemical approach, mix chocolate laxatives with the chocolate chips. This will ensure that the desired affects are actualized.
For added fun, remove the majority of the toilet paper from the staff bathrooms. If this seems too extreme, replace toilet paper with the most unpleasant one ply you can find. Often offices already provide low end toilet paper, so half of the job may already be done for you!
Other options include perfume bombing the bathroom, wetting toilet paper rolls and repeatedly knocking on the bathroom door. Simply locking the bathroom door is always a good trick to have in your pocket. Just be sure you did not have any muffins yourself.