I've been slowly working on a script that allows you to do this (and other tricks) with switches and variables. Here is the link to the current version (version 0.4):
http://forums.rpgmakerweb.com/index.php?/topic/33348-random-encounter-control-version-04-updated-120914/
Just add the notetag <Encounter Switch: 205> to the map you want to be on (replace 205 with whatever switch you want to use, I just used 205 in my game) and turn it ON at game start. Turn it back OFF when you want battles to occur. This way you can also set it up so that some maps have random battles at specific plot points by turning on and off the switches, just as long as you use the notetag on each map affected.
Or, you can use the solution posted above me too. I just prefer the switches as that way I can turn it on and off for individual maps as needed. Admittedly, their solution is simpler too if you are just starting out and don't want more advanced control.
Just make sure to turn on switch 120 at game start if you do use my script. That one is important (until I can edit it more anyways).