Cheap and cheerful you can use the hscript “message” command anywhere you evaluate a parameter or houdini hscript (HDA scripts like onupdate script, shelf button, etc):
In a texptort type:
to see help usage on the command
Then try:
will pop up a dialog that will pause EVERYTHING until you hit that button.
If Python and you want also cheap and cheerful, you can use hou.ui.displayMessage()
In a Houdini Python Shell type:
hou.ui.displayMessage("My Dialog", buttons=("OK",))
Beyond that you are writing your own Python +QT interrupt.