By default, the platform does not have this option. You can use the following CSS code.
Please follow the steps below:
1. Go to BuddyBoss > Theme Options
2. Under Custom Codes, enable CSS
3. Append the following:
.comment-author .avatar, .post-author-details .avatar, .entry-content-wrap .avatar, img.avatar { border-radius:0 }
4. Save changes.
SCREENSHOTS:
Without CSS:

With CSS:
