Certainly they could say no, but I would echo the point that churches do want to participate in the process. They want to be seen by their communities as being active partners in the democratic process, so they would want their buildings used.
The other thing I might say about rental space is that in our experience United Church congregations often rent these spaces at a discounted rate. Elections Canada pays at a good rate, so if you're the person making the decision on who you want to rent to, that may have an influence.