❔ ✅ Stopping any additional subclassing of a class

I've got the following code:

public abstract class MyBase {}

public class MyType1 : MyBase {}
public class MyType2 : MyBase {}

public abstract class MyBase {}

public class MyType1 : MyBase {}
public class MyType2 : MyBase {}

Is there a way of stopping any other classes subclassing

MyBase

MyBase

other than the two here? I need to be able to do these subclasses, but don't want anyone to be able to do any more.