Making peace in the Big room

With a bit of planning and minimal amount of equipment, it's possible
to fight monster generation, to the point that no new monsters appear
on the level for the rest of the game, letting you harvest your herbs
in peace.

In order to achieve the desired effect, using this particular
solution, in this particular level, one needs...

* a breeder.

* 14 doors, forming an enclosed area of 8*5 or 7*6 tiles, or 1-13
doors and a method of digging, enclosing an area of at least 40
tiles total.

The first thing to do is to find a breeder and leave it somewhere,
until you have the necessary equipment. For those who like to do
things the alternative way, and are in a position to do so before
finding a naturally generated breeder from a nearby CoC level (or the
Big Room itself), find/create a corruption trap and lure a regular rat
over it... to generate a chaos rat... which is a breeder.

I'll provide two examples on how to proceed from here, depending on
the amount of doors you're planning to invest in this.

Example 1 (14 doors, no digging):

Choose a corner that would disturb your herb patterns the least, and
create the doors, like this:

#..........
#+++++++...
#......+...
#......+...
#......+...
#......+...
#......+...
#......+...
#......+...
###########

Now, get the breeder from wherever you left it, and lure it inside the
box. Close the doors, so that the breeder won't escape and wait
outside. Protect the doors from random passers-by and observe the
occasional popping/burping sounds.

Once the amount of breeders reaches 40, all monster generation in the
level stops. Start to wander further from the box and kill the
remaining monsters from the level.

Example 2 (1 door, some digging):

At first, it would seem that the big room has no walls to dig, but
upon further investigation this appears to not be the case. You can
dig one tile left, one tile up, three tiles right and two
tiles down, into the wall. This leaves the lower right corner as the
obvious choice to start digging, and the two-tile wide corridor in
the right side allows for getting around the initial breeder without
needing a second exit. Enclose a 40-tile area...

   ####
####..#
...#..#
...#..#
...#..#
...#..#
...#..#
...#..#
...#..#
...#..#
...#..#
...#..#
...#..#
...#..#
...#..#
...#..#
..+#..#
###...#
#.....#
#######

...get the breeder inside and apply the same method as in the first
example.

If everything went well, you're now looking at a completely peaceful,
guaranteed herb level, which is guaranteed to be in a non-corrupting
zone.