For each module, the demo field (readonly, boolean) on ir.module.module determines if demo data will be installed when the module is installed.

Readonly fields can be changed via code.

Try the parameter --without-demo=True and no module will load the demo data. 

You can try also a dict to selectively load deo data (but results can be unexpected because the relation between demo on modules).

Try the parameter --without-demo={'module': True} and only the modules specified will load the demo data.

I reject your reality and substitute my own!

After ten years of "this cannot be done", to toggle demo data for all modules to false:

​UPDATE ir_module_module SET demo = 'f' WHERE demo = 't';​ 

A more selective WHERE condition can be used to limit it to certain modules.

You must create a new instance.

Demo data is a setting that is turned on/off at the time you create the instance. If you tell it to import demo data at creation you cannot later turn it off.