Microblog customize - Force users to post in one category and choose tag

Hello,

I'm trying to customize microblog to let users to post with many many restrictions.

One of them is to force them to choose one specific category only. Currentely microblog dashboard will show the option to choose only the category I want, but still they can post without choosing it.

I would like too to force user to pick at least one of the tags.

Would you indicate me the direction where should I hack to code to reach this results, supposing that there are no options in settings to achieve it?

Thanks a lot!!