Seriously, all of the responses here are good, but they address the symptom of your problem, not the problem.
Your problem is that you don't have a culture of innovation. You need to create a culture of innovation, to do that you need to fix some things.
1) You are doing performance reviews wrong. Don't feel bad, almost everyone is. Fix that.
2) Your workers think that innovation is optional and something that is a bonus. Innovation is an expected part of your job, and if you are not innovating you are not doing your job.
Grishnahk says, "Any innovative ideas I come up will be kept hidden until I'm out of here."
You need to convince people that the only way that they will succeed is if the company succeeds and you need to reward people when they do their job well. (See point #1) The Grishnahks of the world will constantly seek the workplace where mediocrity is tolerated. There are 500,000 new employees looking for jobs from last month alone. Get rid of Grishnahk.
3) Openness. People need to know that you got rid of Grishnahk and why. People need to know that you gave Mark's job to John because John worked harder and contributed more.
Do not tolerate substandard work. Expect innovation as a fundamental core of each persons job, not as a volunteer opportunity. Reward hard work with recognition.
*Portrayal of Grishnahk as a slacker was from a single statement and used for illustrative purposes only.