Digital switching of high voltage analog signal
Easiest way is to use a telecom type relay. 



All you need is a suitable relay driver. 

They're fully characterized for the frequency range you're interested in (and well beyond). 



Added in edit:

Since you need to switch quickly, something like this may work for you: 



At 3ms it may be a bit marginal for 100Hz switching, depending on what exactly you're doing, but the 350usec (typical) models don't look too bad to me. You could always use a series/shunt arrangement if you need to increase the off attenuation.