File creation/deletion is controlled by directory permissions. Unless you use a sticky bit, file ownership doesn't matter.
If you want to restrict file deletion to the owner only you need to set the sticky bit on the directory. I'd also suggest sgid in your case to force files into the correct group,
chown root:grp dir
chmod 3775 dir
I make the directory ownership root, to put all members of grp on an even footing, though you can use any other user for this. All access to the dir will be controlled by grp.
Give that a try, see if it meets your needs.