To mark as obsolete with a warning:
[Obsolete]
private static void SomeMethod()
You get a warning when you use it:
And with IntelliSense:
If you want a message:
[Obsolete("My message")]
private static void SomeMethod()
Here's the IntelliSense tool tip:
Finally if you want the usage to be flagged as an error:
[Obsolete("My message", true)]
private static void SomeMethod()
When used this is what you get:
Note: Use the message to tell people what they should use instead, not why it is obsolete.