Semi agreed with hkanchor.
You need to be more ummm how can I say it, You need to be in control of your own situation and don't let anybody force you to do anything you don't wish to do or feel uncomfortable with.
If you wish to say no then just say it and walk away and don't let anybody treat you like a doormat. Lifes too short as it is and you're better than that.
The Mtr tactic is if they stop and take a look just say noyou're busy and carry on walking, They'll soon get the hint and won't ask you again.
I used to be exactly the same as you and now won't ever get myself in that situation again.