This is a way for admins to mitigate some issues caused by caps if such issues ever arise.
E.g. add this to global level in znc.conf:
DisableClientCap = sasl
DisableServerCap = chghost
DisableServerCap = message-tags
Then these caps will be NAKed to client / not requested from server.
Note that this mechanism doesn't fully prevent a cap from being activated, e.g. one could use *send_raw module to request it from server even when disabled.