Using 4.7.18 and Wordpress 4.7.4, on my contribution page that I want to configure as a membership only sign-up page, I untick the checkbox labelled "Contribution Amounts section enabled" on the 'Amounts' tab of the contribution page set-up wizard. However, after saving the page, if i then go pack in I see that this field is once again ticked.
I've just repeated this on http://wpmaster.demo.civicrm.org and got the same result. It looks like this is a bug.
Update: now logged as an issue here: https://issues.civicrm.org/jira/browse/CRM-20495