Use the Automatic Quota Control to automatically disable your survey after a specified number of respondents have taken your survey. After the quota is reached users coming to take your survey will see the "Inactive Message". You can customize the message that users see also. Click on :

• Login »  My Surveys »  (Select Survey) »  Survey Control »  Security

Screenshot

Note: If you have "Branching" and "Termination" enabled in your survey, the count represents the total number of users who have completed the survey *without* getting terminated ie. gone through the last question of the survey.

If you do not want this, make sure that that the quota is set to zero. Setting the Quota to 0 will disable Automatic Quota Control.