How to prevent a node being saved when using the Rules module?
I would try to use the Conditional Rules module for this. Some more details about this module (from its project page):


  ... provides Rules plugins for conditionally altering execution flow directly inside a group of actions.


So create a rule in which you add these actions:


Conditional (which you add via the 'Add conditional' link at the bottom)
If Condition (which you add via the 'Add If' link to the right of the 'Conditional' action, and 'Condition' is what you mentioned in your first rule already).
Redirect &amp; set message (which you add via the 'Add Action' link to the right of the 'If' action, and 'Redirect &amp; set message' is what you mentioned in your first rule already). 
Else (which you add via the 'Add Else' link to the right of the 'Conditional' action.
Calculate &amp; set value (which you add via the 'Add Action' link to the right of the 'Else' action, and 'Calculate &amp; set value' is what you mentioned in your second rule already). 


Here is a screenprint of a rule with an action that uses Conditional Rules (though only with an "if"-part here, no "else"):