if i understand correctly, you have 240/120v available at the outlet via a 14-30 receptacle but you only need 240v for your brew panel (no 120v required). is this correct? if so, you will either need to replace the 14-30 receptacle with a L6-30 receptacle (to match your existing plug) or replace the plug on your brew panel cord with a 14-30 plug (to match your existing recep). if going the former route, simply leave the neutral wire unconnected at the receptacle (place a wire nut on the end of the neutral wire). if going the latter, you don't need to do anything with the existing recep although you may want to disconnect the neutral at the receptacle anyway, just to eliminate a potential additional path to ground.
and if my above assumption on the existing configuration is incorrect, please describe your existing installation in more detail. in any event, do not tie neutral and ground together at the recep.