You are allowed to delete your activities/comments that you created, and those in your activity stream and in the space where your are the manager.
Change the activity filter to All Activities or My spaces to view all of your activities.
Hover your cursor over the activity you want to delete, then click that appears.
Click OK in the confirmation message to accept your deletion.
As an eXo Platform user, you can only delete your own activities.
If you are manager of a space, you can delete any activities related to this space.