I sometimes star a word because I do not want to learn it. But when I go to "my words" and click on "stared words", the option of delete in the resulting list is not available. Any idea what I am doing wrong?

Starred words were built with the idea of using them to mark words you want to study more, like hard ones or important ones. What would be better for you is when a word comes up you don't want to study, you open the word popup for that word (the magnifying glass on the study page, or the word itself on the starred words page) and click all the red circles of the parts of the word you don't want to study.

Hope that helps!

You can also look the word up on "my words", click on the word in the result, and from there remove it as Scott says above.