Perhaps you could use the Advanced Filter?
In Sibelius 8, it's under the Home Tab. Shortcut key is Alt+Ctrl+Shift+F.
It's not super intuitive, but try using it to select all the clefs. Something like this:
Note that you can select multiple clefs by holding down Ctrl and clicking. I note that there are duplicate percussion clefs, so you may want to select them as well.
This also assumes you have everything selected- the filter filters the selection, unless you change "Find in" to "Whole score".
Once you've run the filter, all the clefs should be selected. Then just press delete to remove them all.
I'm assuming the objects are actually clefs, and not a symbol or something weird. Regardless, with some experimentation, you should be able to use the filter to just select those objects, and then delete them.