You have to edit the file events.txt for this. The following entry in this file should prevent ALL civs from negotiating:
@IF
NEGOTIATION
talker=ANYBODY
talkertype=HumanOrComputer
listener=ANYBODY
listenertype=HumanOrComputer
@THEN
@ENDIF
But it seems that this works not in every scenario, in another thread, for instance Nemo has said that you need to modify the entry above like this (if I remember exactly):
@IF
NEGOTIATION
talker=ANYBODY
talkertype=HumanOrComputer
listener=ANYBODY
listenertype=HumanOrComputer
@THEN
@ENDIF
Note the blank line between THEN and ENDIF.
You can change the talker, listener and so on to make such an event only for some civs:
@IF
NEGOTIATION
talker=ROMANS
talkertype=HumanOrComputer
listener=GREEKS
listenertype=HumanOrComputer
@THEN
@ENDIF
In the example above, only Romans and Greeks aren´t allowed to have negotiations, but you need a second event, to complete this for both sides (simply exchange in this second event the names of the talker and listener civ)
