If you have just started modeling, especially if you joined a networking site for models, you will get several offers from people who you may not want to work with before you get offers from people that you do want to work with.
The people that you would want to work with are probably those that you should approach yourself, because chances are they get so many messages from people wanting to work with them that they do not have the time to send you a message, or even look at your profile.
However, if you get a message from someone that you don't want to work with, or whose concept/idea/proposal does not appeal to you, don't be afraid to tell them that you are not interested. If that person is mature, he/she will take it with a grain of salt and won't contact you again with the same proposal.
If he/she chooses to have a hissy fit about your response, and starts telling you about the "world of opportunities" that you have missed by choosing not to work with him/her, or starts writing you nasty messages in response to your rejection of their offer, then the best thing to do is to block the person and stop all future communication. By nipping this in the bud right then and there, you are saving yourself a world of weirdness/trouble. If that person reacts that way to a simple "Thank you for your offer but I am not interested", think of how they would react to your future communications where you happen to disagree on something.
The person who sent you the proposal to work together should understand that rejection is part of life.