Durable unsubscribe shouldn't block these changes - see Durable Unsubscribe - Marketo Docs - Product Documentation .
If these methods detailed in Docs aren't working in your instance, then it's possible that either updates to that field have been blocked or a smart campaign has been set to change it back. I'd recommend looking at the activity log for a lead that you can demonstrate this on and see what the data change history suggests.
Agree with Grace, check the field settings if you are blocking it from updates.
Top right corner click Admin > Field Management > Search for "Unsubscribe" or if you have a different name for it > On the right, click the field > Field Actions > Block Field Updates > Uncheck anything you want to allow it to update.
+1 on Grace, the problem is elsewhere.
Please note that in many countries (All of the EU for instance), resubscribing someone manually or with a smart countries is illegal unless you can prove subscription was requested by the person herself.