However, there is a workaround as mentioned in this blog post but this will remove the field from visibility altogether so really depends on your requirements:
hi scott we don't disable this field as well as can't hide this field from layout as it plays dominent role in salesprocess and analytics/decison making. but you can hide it through visualforce page but in that case you have to define a static value for closed date , if field remain empty it doesn't allow user to insert record.
Thank you. We just have a unique situation in how we are using the Oppty Object. We put a default date in for 2099. Because I could not find a way to remove the close date from view. Oh well.
https://success.salesforce.com/ideaView?id=08730000000BqEzAAK
http://www.simplysfdc.com/2014/02/salesforce-how-to-hide-probability-field.html
we don't disable this field as well as can't hide this field from layout as it plays dominent role in salesprocess and analytics/decison making.
but you can hide it through visualforce page but in that case you have to define a static value for closed date , if field remain empty it doesn't allow user to insert record.