There is a game option that disables barbarians entirely, but no "reduced barbarians" game option.
Setting that aside, barbarian spawns are controlled by the size of the landmass you are on, and specifically the number of unclaimed tiles remaining on it (tiles that are not within any civilization's cultural borders). The game will add all of those unclaimed tiles up, and divide it by the difficulty-level-dependent unowned tiles per barbarian unit limit. That tells it how many barbarian units are "allowed" to exist simultaneously on your continent. For example, on Monarch the limit is 40: one barbarian per 40 unclaimed tiles. If your continent has 800 unclaimed tiles, it can support up to 20 barbarian units on Monarch.
Each turn after barb spawns start, if there are *fewer* barbarians on the continent than the limit then it will add one barb, plus 25% of the missing barbs (rounded down). If the continent above had just 16 barbarians, it would spawn 2 barbarians: (20-16)/4 + 1. Filling up more land will reduce barbarian spawn rate, although it will also stretch you a little thinner trying to cover all the land you have. Killing fewer barbarians will actually reduce the spawn rate - once it hits the cap, new spawns will be only replacing losses. Although this is more of a preventative thing - don't go hunting barbs out in the unclaimed areas most of the time, try to let them come to you instead; this means each individual barbarian moves further and lives longer before dying, which means slower replacements.
More useful is trying to direct those barbarian spawns at your neighbors, or at least towards just one part of your empire where you can put strong defenders to kill them. Barbarians will be placed in random eligible tiles on the continent; to be eligible, a tile must not be visible to any civ, not be within 2 spaces (diagonally, vertically, diagonally) of an existing unit for any civ, and not be a mountain. So if you arrange units around the unclaimed land such that the 5x5 box centered around them covers all the nearby land tiles, no barbs are going to be spawning in those regions. Which usually means fewer spawns near you and more spawns near your AI neighbors, who will then go attack the AIs instead of bothering you.
This is an idea called "fogbusting" and there's a lot of really useful examples of it floating around this forum if you want to search (it's practically an essential tactic for people playing on the top difficulty settings, where barbarians can easily ruin your whole game if you don't handle them right).
If you don't want to worry about adjusting strategy and really just want a simple mod to reduce the number of barbarians that can be spawned at once (and the number that will spawn per turn), you can do it yourself if you don't mind changing a number in the game files.
You're looking for a file called "Civ4HandicapInfo.xml"; it will be in a folder somewhere like Sid Meier's Civilization IV Beyond the Sword\Beyond the Sword\Assets\XML\GameInfo.
Within that file, you want to find the difficulty level you prefer to play on, and the "iUnownedTilesPerBarbarianUnit" entry for that difficulty. For example, if you prefer playing on Monarch if you'd look for <Type>HANDICAP_MONARCH</Type>, then look down a couple dozen lines from there for "<iUnownedTilesPerBarbarianUnit>40</iUnownedTilesPerBarbarianUnit>". Just change that number 40 to whatever you prefer; higher means fewer barbs. 80 would mean half as many barbs; 20 would mean twice as many.