Inside Hazard Utils you will find a Dynamic Brush button that will switch between On and Off you brush dynamic feature.

Optionally you can install the OnOff version that will place 2 buttons to perform the swith mode.

To Install just paste the script on your current Zbrush installation inside ZStartup/ZPlugins

You can assign a hotkey by Ctrl+Alt+Click or enable customize in Preferences and place the button wherever you wish by Ctrl+Alt+Drag