Objective-C
NSMutableAttributedString *attributeString = [[NSMutableAttributedString alloc] initWithString:@"Your String here"];
[attributeString addAttribute:NSStrikethroughStyleAttributeName
value:@2
range:NSMakeRange(0, [attributeString length])];
Swift
let attributeString: NSMutableAttributedString = NSMutableAttributedString(string: "Your String here")
attributeString.addAttribute(NSStrikethroughStyleAttributeName, value: 2, range: NSMakeRange(0, attributeString.length))
Then you can add this to your UILabel:
yourLabel.attributedText = attributeString;