You are an independent contractor, meaning you have total control over which gigs you take and which you pass on.
When you send a proposal, you are telling the client "I am available and want this gig at this price." Once the client pays, the gig is Confirmed. You cannot unilaterally cancel a confirmed gig without consequences (see Cancellation Policy).
If a gig isn't right for you, or if the budget is too low and the client won't negotiate, it is better to decline the request promptly than to ignore it.