Adobe PDF prohibit unchecking of check box

I have an Interactive PDF form. On this form I have a small group (7 options)of checkboxes. 2 of the boxes are directly associated with each other (one way relationship)

These 2 checkboxes are Network and Infrastructure (dependent upon Network).

The scenarion is the following:

• If Network is checked then Infrastructure needs to be checked as well. (This part is complete)
• If user tries to unchecked Infrastructure, then they will not be able to unchecked it without first unchecking Network, but I want to explain to them why, think alert or something. ( this i am having problems with)
• Infrastructure can be toggled on/off any other time Network is not checked. Got this covered)

I currently am using the following script on Mouse Up:

 Var f = this.getField("cr_infrastructure");

If (event.target.isBoxChecked(0,true))

{ f.checkThisBox(0,true);}

Else

{f.checkThisBox(0,false);}