This happens to me when one or more of them opens it with ACCDB file instead of ACCDE.

Yea sounds like someone is maybe using a shortcut to the same front end file instead of having their own copy.

Yes, some object somewhere in your database is referencing an Excel file. Make a backup copy of your database and start deleting objects until you figure out which one is causing it. When you find out the object that's causing it, like a former module, start deleting controls off of that form or code out of the module until you figure it out. The process of elimination is the easiest way to find these things.

Also Excel dont work good in a shared enviroment.  I have to copy data to a temp table but you run into the problem that if someone else did an update you may over-write the data...