How to avoid users to be redirected to PayPal if they want to pay offline?

I offer offline and online payment options.

When users select to pay offline, they are naturally still checking the online payment options. When they submit form it automatically sends them to PayPal; since they selected "offline" previously, they close the PayPal window and the form never properly submits.

I have a note on my form to leave the "Pay Online" section completely blank if they are making offline payments, but lots of parents still naturally select the online payment options, are pushed to PayPal site, close the form, and their form never submits.

Do you have a solution to this issue?

You may hide the payment field instead of asking the user's to leave it blank, this way you avoid them to still fill in the amount. 

Here's a guide that will help you doing so: https://www.jotform.com/help/316-How-to-Show-or-Hide-Fields-Base-on-User-s-Answer 

You may set  up a condition to hide the payment field depending on the payment method they want to use. 

You will also need to set the form to clear the hidden fields values, you will find the option under the form settings page: 

I hope this helps.