Acknowledge and appreciate them
Although it can be difficult, try not to react in a condescending way. As an article on the TED Ideas blog advises, it is better to appreciate the person's insight so they do not feel too wronged. Recognition and attention is usually what these people are seeking.
Explain that you do not need their opinion right now
Just because you appreciate the person's insight does not mean you will take their opinion into account. If, for example, the employee suddenly starts talking during a team discussion, explain to them calmly, clearly and concisely that there is no room for their opinion right now and you will let someone whose expertise is more relevant speak.
Do not let yourself or other colleagues be interrupted
If the person still tries to assert their opinions by interrupting the debate, you must intervene. Do not let yourself or your colleagues be interrupted, and set clear rules for internal communication.
Take into account pieces of advice and comments that are relevant
Do not always try to deflect the person at all costs. It may well be that some of what they say is relevant. Always try to extract any comments that are useful and productive, and make them happy by taking their opinions into account.
Focus your attention on other team members too
An employee who is constantly and loudly forcing their opinions on the team often draws too much attention to themselves. Do not be distracted by them and pay at least as much attention to other workers too. Their opinions and comments are often more interesting than the speeches of the know-alls.
-mm-