It is currently not possible to disable/hide the local environment (FNAT/FUSER) when working with a SPoD environment. This applies to all kind of servers.
The local environment is completely independant from any mapped environment. If you wan to secure your local environment you have to install Natural security on the PC
To use a centralized FUSER the Network server installation could be used for having a shared FUSER and single parameter profile, or a variation of that idea could be implemented too.
If the Network Server option is used all the main files are stored in a single repository, then each individual developer PC only has to run a quick little setup.
When this is done everyone is using the FUSER defined on the machine where the Network Server installation was done as well as the Natural parameters defined there, so if someone changes the default config then everyone gets the change.
If everyone is using the same FUSER then Windows security can be used to limit access to certain library folders to prevent/limit access.