One of the best techniques for lowering motivation: Give the employee an important task, make him work on it 24/7 and state how vital it is to the company.
Then management will rethink the task and decide it was not necessary anymore. Manager forgets to inform the employee.
Most important part: several weeks later, as the employee is getting extremely stressed and exhausted from the task, the manager should make a side comment that the task was cancelled several weeks ago. Then, as the employee is outraged, the manager will calmly say: "What? You were still working on this? Are you kidding? EVERYBODY knows that this task was cancelled! Man, you should try to get more informed if you want to be appreciated round here!"