You can try adding some salt to your solution of nanoparticles. But as Sagar suggested it would be better to use centrifugation since the salt-induced precipitation may not be reversible.
Thank you Sagar and Philips, Indeed I need a method to prepare large amounts so it is better to get a preciptate rather than a colloidal solution.
I have actually tried your suggestions before, but the strange thing that some times the method which gives me usually milky appearance give me precipitate !!!! so I can separate it easelly