To get rid of quizzes you no longer need, go to your Quizzes tab:
Find the quiz you'd like to delete and check the box to the left of the quiz. Once selected, click on the Delete icon. The quiz will then go to your Trash section.
On the Socrative Free plan, quizzes in the trash do not count towards your quiz limit.
With Socrative Pro, you can restore quizzes from your trash that you'd like to use again. Learn how to restore a quiz here.