Either via:
NSString *immutableString = [NSString stringWithString:yourMutableString];
or via:
NSString *immutableString = [[yourMutableString copy] autorelease];
//Note that calling [foo copy] on a mutable object of which there exists an immutable variant
//such as NSMutableString, NSMutableArray, NSMutableDictionary from the Foundation framework
//is expected to return an immutable copy. For a mutable copy call [foo mutableCopy] instead.
Being a subclass of NSString however you can just cast it to an NSString
NSString *immutableString = yourMutableString;
making it appear immutable, even though it in fact stays mutable.
Many methods actually return mutable instances despite being declared to return immutable ones.