how can i prevent my phone from connecting to a certain network?

You are in control of the Wi-Fi hotspots you connect to.  If you want to disconnect from a hotspot or remove it so it is not connected to in the future:

1.  Open the WeFi Pro application by selecting it in your device’s application list.  Access to your application list varies by

 device.  If you are unsure how to access your list of applications please consult your manual.

2.  Press your device’s Menu button/key.

3.  Press Networks.

4.  From the list of Wi-Fi networks press and hold the hotspot you want to disconnect or remove.

5.  Select to Forget the hotpot.