How can I prevent updates?

The modem works pretty well with factory settings.  When it updates it is imediately noticable and my internet speed goes form 15-20 mbs to 1 mbs.  Restarting almost never improves performance.  Factory reset does the trick.  I have to do this sometimes twice a day. 

Is there a way to prevent updates?  They seem to be messing me up. 

Thanks!