There is no definitive reason why unreachable statements must be not be allowed; other languages allow them without problems. For your specific need, this is the usual trick:
if (true) return;
It looks nonsensical, anyone who reads the code will guess that it must have been done deliberately, not a careless mistake of leaving the rest of statements unreachable.
Java has a little bit support for "conditional compilation"
http://java.sun.com/docs/books/jls/third_edition/html/statements.html#14.21
if (false) { x=3; }
does not result in a compile-time error. An optimizing compiler may realize that the statement x=3; will never be executed and may choose to omit the code for that statement from the generated class file, but the statement x=3; is not regarded as "unreachable" in the technical sense specified here.
The rationale for this differing treatment is to allow programmers to define "flag variables" such as:
static final boolean DEBUG = false;
and then write code such as:
if (DEBUG) { x=3; }
The idea is that it should be possible to change the value of DEBUG from false to true or from true to false and then compile the code correctly with no other changes to the program text.