Can you contact Yahoo if a user is bullying you ?

There isn't a dedicated way to report a user. You can report their posts on Yahoo Answers, but that's it. You can block them, if they aren't hiding behind the Anonymous feature. Otherwise, ignore them or go Anonymous yourself, so they won't know when a post is one by you.
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Move your question to Yahoo Products>Yahoo Answers as it's currently in Yahoo Products>Yahoo Mobile and not really applicable to this subcategory (and thus decreases your chances of getting many useful answers).   You can move your question by clicking Edit (pencil icon for mobile)>Edit category just beneath your question, then selecting the category in which it is better suited. 

Contact Yahoo Help: https://io.help.yahoo.com/contact/index?page=topics&locale=en_US&y=PROD_ANSW

just report and block em

All you can do is block them or post as anon so they don't know it's you.

https://help.yahoo.com/kb/answers

Step 1: Log off
There is no step 2.