How to disable one checkbox in a group based on text box having value
You can use jQuery in the webform to hide/display the checkboxes based on the contents of the membership id box.  A simple implementation could just check whether the membership box is blank, but then any random stuff would show the member options.  A next step would check the membership id is of the right format.  Even better would be using the API to check the validity of the id entered.

That will let you display what you want but does not prevent a determined user displaying the hidden options so you should probably add a validation rule as well.

Also, if the person filling out the form is an existing contact, you could use the auto-populate feature for existing contacts to get the webform to display the membership number directly.