Follow the steps below to hide the skip survey button:
  1. Go to Content.
  2. Locate your survey.
  3. Select "Edit".
  4. Select Step 2  Include Questions
  5. Select "Add questions" and choose Caption
  6. Step 3. Question Text (switch over to use Plain Text) and input the following code:
<style type="text/css">

INPUT#ACTION_CANCEL_RESPONSE_SUBMIT.Button
{visibility:hidden;
}
</style>

Click Save and Finish.