I have seen a few people ask this. I never figured out why people would want to.
You have to call tech support to remove the option. You cannot do it on your own.
It's common to deactivate voicemail people do it all the time to use another service such as Google Voice.
You can set your conditional forwards to a number that won't answer. You used to be able to get Tech Support to disable DCF, which allowed the customer to turn voicemail off and on but I've been told that they won't do that any more.. So, you're stuck in the-customer-is-an-idiot mode.
In the case of Google voice you are not deactivating voicemail but forwarding to another number other than the T-Mobile voicemail system. In that case you just change the conditional call forwarding to the service you want to use instead.
I was unaware that they didn't do that anymore. It has been a while since I have heard of anyone requesting it.
Depending on what kind of phone you have:
IF you have an Phone:
Step 1: Go to your home screen and tap on the keypad icon at the bottom.
Step 2: Once the keypad is launched, type ##002# and then click on the Call button. There is no cause for worry since the iPhone will not actually make the call, but will display the settings of your voicemail.
Step 3: You will be presented with three different messages
* Voice Call Forwarding
* Data Call Forwarding
* Fax Call forwarding.
Do not do anything, but simply tap on the “Dismiss” tab at the bottom of the screen. You will have successfully disabled your voicemail for all types of calls.