Want to offer free hosting! Need some information.

I want to offer free PHP/MySQL hosting to my website visitors but am worried about it being used by spammers to send out emails and to run email harvester scripts.

1. Is it possible to limit the number of emails being sent?
2. Is it possible to force the user to send out emails only to specified email addresses?
3. Are the above options possible on shared hosting or do I need to go for VPS?

Thanks,
Jatinder

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