What version of APEX are you using?
In 4.2 (I'm using Theme 25) you can set a region as a Modal region that pops up when a button is clicked.
1) In your page, create a new HTML Region, call it something like "Confirm Action". In the Region Definition, set the Display Point to be "After Header", and the Template to "Modal Region".
Also, set the Static ID of the region to 'confirmregion'
2) Within your new "Confirm Action" region, create a new Region Button, call it something like 'Confirm'. (You can also create a button called 'Cancel')
4) Next, change your PL/SQL Page process to only run when the new 'Confirm' button is clicked.
Sorry I could not recall which version this became available.