So I have a few people I'd prefer not to get messages from in future, basically I'd like a 'fresh start' with my imessage account. If I change my Apple ID to a new email address, will those people still be able to imessage me, or will (as i am hoping) they be unable to contact me as the email address associated with imessage that they've been using would now be out of date?
When you change your Apple ID - the actual text you enter before you enter your password - then it is similar to changing your telephone number. So, yes you have a new account at that point with respect to people messaging you.
Yes, the bill still comes to you and it rings the phone you used to have, but people need to discover or be told your new "digits" before they can "call" you.
Now - there are two qualifications: