Everyone with whom I have talked describes some project they have been on where it seemed that the project never ended. Even if they were switched to another project, the initial project seemed to go on and on of its own volition. Almost everyone called this a part of their "project from hell." There are many reasons why projects don't end. Some projects can go on and on because of funding issues. In many companies, it is easier to get funds to do a project than it is to get operating funds. Even when the project is actually complete, it is sometimes easier to request ongoing funds even though the product of the project is completed. When the project has been done and the actions have become a part of the operating system, the project is over. It is not unusual for a group of people or a segment of a company to want to continue the process they have started, but in fact it doesn't help the company to continue to call an operating process a project.
How do you stop this? At the beginning of the project planning process, the project manager needs to work with all of the stakeholders to get agreement on what a complete project is. There must be a well defined deliverable that is the final output of the project. Failure to define project completion will cause the project to drag on and on. In one organization for which I consulted, several so-called "projects" had been running for years. In fact, the projects were part of the overall operating system and required some maintenance, but they weren't true projects.