If you are certain you are not accepting an offer you have received from a firm, you should let the firm know immediately, either by phone or by email. There is no benefit to you or the firm to wait until your offer expires if you know you are not going to accept. This allows firms to make offers to other students, and firms will appreciate your transparency and communication. It will also reduce the number of deadlines you are keeping track of, and allow you to focus on the firms you are most interested in.
The emphasis on open communication ensures a smoother process for both students and employers.