I use reusables a lot with a lot of on page load workflows. The best way I found to stop that is to add a only when clause to that workflow that is the same as the reusable.
For instance, I have a reusable called Team Item panel. It’s only visible when the URL contains &mode=team and &item is not empty.
I copy that conditional to the workflow.
The red only when indicates that it evaluates it as false and it doesn’t run the full workflow. But yeah, I know what you mean, I wish it wouldn’t even do that. I don’t think that’s possible.
Maybe you could turn that around and make the page – not the reusable – trigger the reusable’s workflow.