Inside left4dead2/ems/breakmelees is 2 files, durabilities.cfg and settings.cfg. The durabilites file has a list of all the melees and the amount of hits they can do before they break which you can change to your liking. As for the settings file:

givepistol = 1 //default 1, whenever a melee breaks, if this is set to 1 the survivor is given a replacement pistol

vocaliselines = 1 //default 1, whenever a melee breaks, if this is set to 1 the survivors will play voice lines expressing their contempt

textlines = 1 //default 1, whenever the durability of a melee gets very low, if this is set to 1 some messages in the chat box will appear informing you as such