NSUserDefaults+.swift

import Foundation

extension NSUserDefaults {
    func removeAllKeys(except except: [String]? = nil) {
        guard let appDomain = NSBundle.mainBundle().bundleIdentifier else { return }
        
        let defaults = NSUserDefaults.standardUserDefaults()
        
        var dict: [String: AnyObject]?
        if let except = except {
            for key in except {
                if let value = defaults.objectForKey(key) {
                    if dict == nil { dict = [:] }
                    dict![key] = value
                }
            }
        }
        defer {
            if let dict = dict {
                for (key, value) in dict {
                    defaults.setObject(value, forKey: key)
                }
            }
        }
        
        defaults.removePersistentDomainForName(appDomain)
    }
}

// Usage:
NSUserDefaults.standardUserDefaults().setObject("One", forKey: "key1")
NSUserDefaults.standardUserDefaults().setObject("None", forKey: "keyN")

NSUserDefaults.standardUserDefaults().removeAllKeys(except: ["keyN"])

print("key1:", NSUserDefaults.standardUserDefaults().objectForKey("key1"))
print("keyN:", NSUserDefaults.standardUserDefaults().objectForKey("keyN"))