How to stop Avast from running Full-Scans

I have a custom scan scheduled with Avast to run once a week overnight, but when I check my scan history, it also shows a bunch of Full-Scans that have happened in between. Is there any way to stop these from happening? All I want to run is my custom scheduled scan.

Thank you!

What Avast? Version number?
What OS? What service pack?
Any other a/v’s installed or removed?