This can be done by providing a custom CellViewerManager that displays an image instead of a CheckBox for a boolean column. In fact, we have a topic in the help documentation that shows how to derive for the CellViewerManager class for this specific need.
In this class, you can simply set the FalseImage member to null in the constructor ( e.g. BooleanImageTextViewer.FalseImage = null; ) and set the text to an empty string in the GetTextCore override ( e.g. return string.Empty; ) so only the true icon is displayed, with not text.
Here is more details on how to work with CellViewerManagers.