When you borrow an ebook from a public library, it offers you to download an .acsm file. Do not confuse .acsm files with your actual eBooks. .acsm files are just links to your eBooks that allow software like Adobe Digital Editions to download the actual book. When Adobe Digital Editions opens an .acsm file, it will automatically download the PDF or ePUB file, which is the actual eBook.
So you need to remove DRM from ePub or PDF files, not from .acsm files.
Download freeware
ebook DRM removal tools archive including DRM removal tools for epub and pdf files. And try to use
Calibre plug-ins first.