PDFs sent to us to print...

We get PDF's from all over, often they are made up obviously using greyscale images, along with spot color, but at some point file gets changed to color, so that when placed in Indesign, they won't separate to black w/spot color. Is there anything that can be done at this point, or must we always get back to advertiser to straighten it out. Most often we don't know what software used, or their skill level. (or ours...!)