Once you've posted a comment on an idea, it's possible to delete it. Just hover over the idea and click the 'x' icon in the far right hand side. You'll be asked to confirm the deletion. Once approved, the comment, and all of its associated interactions, such as likes, will be removed permanently permanently from the system.
Any associated notifications or activity history relating to the comment will also be removed.
If you're an organisation admin, you also have the ability to remove the comments from any user across your organisation, in exactly the same way as outlined above.