Hello AndyGrayson,
Thank you for using Apple Support Communities! I understand that you're wondering if there's a way to prevent your MacBook Pro from charging through the connected LG UltraFine display when the Mac is disconnected from its charger. I would be happy to help provide more information to consider.
The lithium battery in your Mac is designed to count a full charge cycle only when it has discharged 100% of the batteries total capacity. If it works best for you, leaving the Mac permanently connected to a charger will not negatively impact the battery life with the way charge cycles are designed to work. You can learn more about this feature by visiting the following resources:
If the Mac is connected to another device capable of charging it using a Thunderbolt 3 port, it is expected behavior for the Mac to charge from that device when not plugged in to an AC adapter. There is not a way to disable this behavior:
Charge your MacBook Pro with Thunderbolt 3 - Apple Support
Take care.