Best How To :

First: Don't do any of this. Structure your program some other way. If you described to us why you think you need this behavior, we could potentially have told you how to achieve it otherwise.

Getting down to the question: If you wrap your block in a loop, you can use break to exit early:

for _ in once; do
  if true; then
    echo "in the loop"
    break
    echo "not reached"
  fi
done
echo "this is reached"

Alternately, you can use a function, and return to exit early:

myfunc() {
  if true; then
    echo "in the loop"
    return
  fi
  echo "unreached"
}
myfunc
echo "this is reached"

Alternately, you can wrap your loop in a subshell (though this will prevent it from doing other things, like variable assignments that impact code outside the subshell, as well):

(if true; then
   echo "in the block"
   exit
   echo "unreached"
 fi)
echo "this is reached."