I have tried to validate my site using the W3 validator, but to my surprise it has given a lot of errors related to K2. The anchor tags inside the rating block have invalid 'rel' values, according to the validator.

You can see it here: validator.w3.org/check?uri=http://demo.getk2.org/item.

What should I do about this issue except having to remove all these tags manually?

This won't affect your SEO ranking or functionality in any way.
If you remove the rel attribute the rating will not work.

Well, all right, I only wanted to make my site HTML5 valid. I hope this little thing will be considered in the next release. Thank you for your answer!

You 're welcome jozsefke.