the same way you brake now, With the mechanical brake. you need to use a Y adapter to split the throttle channel
You can also look for a way to fix the one way bearing to the shaft, or lock the 2 speed crown to the gear. (with the fixed gears, you won't have got a 2 speed anymore)
I have successfully locked secong gear in the stock Revo transmission and it works well. This thread talks all about it, but below is a picture of the modification I used to lock it. This allows me to use reverse and motor brake (no brake servo). There is another member, b0gh0s, who has an adaptor to lock it as well.