You are only able to delete a comment you made on btwb.
Click into the comments section of a workout post. A "trash can" icon will appear next to comments you've made. Click on it to delete the comment.
To delete a comment, go to the member's workout you commented on and select "Comment" on the bottom of the WOD.
Scroll down to the bottom of the page. Next to the comment you will then need to select "Delete". This will remove the comment.