Blocking unwanted WiFi

Hi,

I am a network administrator in my company, we have a wireless setup with particular timings for the users to access the internet.

My problem is that i get alot of unwanted and unsecure wireless signals from outside,to which some of our employees connect to access the internet and thus the security of my nework is at stake. I am looking for solution which can block these unwanted signals ,and stop them from displaying on my employees laptops.

Please help me in this matter, a hardware based blocker would be efficient enough i guess.

Awaiting your replies.