Hi May,
You did not provide enough information to offer constructive suggestions. Do you mean your responses to an essay or manually graded question for a student? Do your really mean delete or mean hide them from students after some period of time. Without knowing why you want to do this, it is difficult to offer other suggestions.
Obviously you can go back in one by one and delete the content of each one. That could be tedious.
As the web site administrator, you could write a query in mySQL to delete all the responses. Not something I would like to do as a general rule.
Under quiz administration>review options, you can do all sort of tricky things to hide things from students after they have take a quiz.
Chris