This is normally done by using a 'factory' tool, to reformat and split the USB into two 'devices' - one is the USB rd/wr part and the other is a CD-ROM part which is read-only.
So you need to find an 'MP-Tool' that will work with your type of USB flash drive. See here.
P.S. Windows can only access the first partition on a 'removable' type of USB flash drive - so you can't partition this type of USb drive.
You could find a USB drive of the 'Fixed Disk' type and partition this as two NTFS partitions - then set one volume as Read-only using DiskPart and set file permissions on all the files on the whole volume using Windows - Security - Permissions on the drive letter (remove Full Control, Modify and Write) - see my blog here for details. It doesn't prevent it from being wiped and reformatted using a tool like RMPrepUSB though.