Cannot clear dynamic_avis folder

I have an up-to date website, and I found that the dynamic_avia folder has grown so much that is ran my server out of drive space. Someone who was given access to the site enabled css and js compression. I have disabled those features and enabled the delete old JS anc CSS option, but the files are still there. Is it possible to delete and re-create the folder? Or do I need to back up my custom css first? I don’t have direct FTP to the site, but have access to the server via RDP.. When I try to delete the files from the command line, I get an invalid argument error.