You may report a comment for spam on Instagram, if you find it spam, scam, or abusive. You can also remove a command under you picture / video, or your post under others' photos or videos.
Do as follows to either report post as spam or delete them (You can only delete your posts, or the posts under your photos or videos):
1. Open Instagram for Android and tap the picture you would like to delete its comments
2. Scroll down and tap the Comment icon
3. Tap the Pencil icon on the top right
4. Now an exclamation icon appears next to the posts (for your posts, you should see a cross icon)
5. Tap the icon to either delete them or report them as spam
6. Choose the action: Delete, Delete & Report for Spam, Report for Spam