Here is the official word from Instagram:
Hi Instagrammers! We’ve appreciated all of your feedback on spam comments, and we want you to know we’re working hard on this to make sure spam doesn’t take away from your Instagram experience. There’s no quick fix, but we have a team of engineers working every day to tackle the issue and we hope you’ll notice their improvements. If you do see spam, please delete and report the comments to us, or visit that user’s profile, tap the button in the top right corner and select “Report for Spam.” Thanks for helping to keep the Instagram community a great experience for everyone!
The good news is that we have noticed a significant reduction in comment spam and also fake accounts following users.
Great work Instagram team in keeping the network free from these trouble makers.