I would like to somehow lock an object so that when I "Save for Web", that particular item on artboard is not included in export but everything else is?
In most cases, it is not possible to have something visible on an artboard and have it be ignored when exporting or saving.
Guides are an exception.
If you just need a visible indicator in the file, you can turn practically any shape (including text) into guides. These are visible in AI, but nowhere else.
Template Layers also have some use.
If you set the objects you do not want to export/save on a single, combined layer, then set that layer as a
Template, the template contents do not export/save when using
Save for Web.
Objects do however take up space. So the objects on the template layer won't show, but the space where they are located is factored into saving/exporting. In other words, they are invisible visually, but their dimensions are still used when calculating width and height upon exporting/save for web.
The red line indicates the
Save for Web size without clip to artboard on. So the space for the red rectangle is seen and used, but the rectangle itself is not visible.
Using template layers this way seems to work for Save for Web or saving to PDF. I can't comment on other save/export methods.