It looks like you're using the Run Continuous button because there are no loops in your code. You need to put a While loop around your code so that you can control when the loop stops.
The reason your popup is showing every time is because of the green boolean wire to that True case. The boolean will be True when the string is not equal to an empty string. The case you have shown outputs an empty string, but I'm guessing another case does not. Look through the different case structure cases to figure out when a string is being output that will cause the dialog box.
You need to understand what's going o in your code in order to fix your code. The best way to unerstand is to learn how LabVIEW works first.
"Give me six hours to chop down a tree and I will spend the first four sharpening the axe." - Abraham Lincoln
Here are some free training tools primarily focused on LabVIEW and NI hardware to help get started.
NI Learning Center
NI Getting Started
-Hardware Basics
-MyRIO Project Essentials Guide (lots of good simple circuits with links to youtube demonstrations)
-LabVEW Basics
-DAQ Application Tutorials
-cRIO Developer's Guide
Learn NI Training Resource Videos
3 Hour LabVIEW Introduction
6 Hour LabVIEW Introduction
Self Paced training for students
Self Paced training beginner to advanced, SSP Required
LabVIEW Wiki on Training
