You should consider users' actions independently of their prior contribution.
If a user is, in excess, causing problems within the community, then the appropriate action to take is to escalate punitive responses, as you would with any other user. Their prior contributions and/or reputation are not a shield against moderative action. This being said, if you know them to be a positive and reasonable contributor, consider that they may be more amenable to hearing things directly from you through a private mode of communication. However, this doesn't always work, even with experienced or patient users.
That being said, be careful with this. It's very easy to be dragged into action simply because they're getting on your nerves. With such a user's posts, take a moment to make sure their contributions are really worth action. However, don't be shy to take action.
In the case of Stack Exchange, this might mean a warning, a moderator message, and eventually suspension. In the case of IRC, this might mean a verbal warning, a kick, a temporary ban, and eventually a permanent ban.
The one exception to this occurs when the user is a critical member of the community - so much so that public punitive action would cause problems within the community. In this case, permanent action isn't advisable, but short temporary action may be.