These comments confuse me in that I cannot determine exactly what you want. Is removing the Drive Letter sufficient for your needs and you just want a way to access the partition after you do, or do you need it hidden in a different manner?
More information might also be helpful. For instance, your options might be different if this is an MBR or GPT drive. Some attributes in MBR will apply to the entire drive, where GPT might be able to apply the attributes to just a single partition.
If you want to access a partition without a drive letter, you can create a link to it in a visible partition, or use the partition's Identifier. The Identifier might be found by using the mountvol command in an administrative command prompt window. You take the ID and paste it into a run box to access the partition. I have no MBR partitions, so I cannot check it, but the GPT version works fine.
Currently, I have a "Recovery" partition I set up that is only visible in Disk Management and a drive letter cannot be added unless you use Diskpart. Is something like that what you want.