There are plenty of ways to reduce power to the 5V blower fan, or even place it under software control. However, if higher temperatures are desired, increased heater power would be a more logical approach. Keep in mind that hotends are specifically designed to provide a sharp transition from cold to hot. In engineering terms, infinite heat sinking would be ideal (infinite stream of air across the cooling fins). In fact, water cooled hotends are not uncommon.
Now, if you are determined to try this, there are a lot of ways to reduce 5V blower fan power.
Simplest is a resistor. Measure the current draw, determine the effective running resistance of the motor, and chose a resistor of comparable value. However, a dumb resistor will not allow the fan to draw extra current at startup, possibly leading to random, unpredictable, failure.
Next simplest would be an LM317, three terminal adjustable voltage regulator. While this is better than a resistor, low voltage settings may not allow sufficient voltage for reliable startup.
Software control from the RAMBO board is also possible. Usually, there is a case fan which runs whenever the stepper motor drives are engaged. For times when the hotend is active, but stepper motors are not, the “M42” G-Code can be issued. Cautiously, an LM7805 linear regulator could be used to share the 12V case fan power with the 5V hotend blower fan. However, if for any reason the case fan is switched off while the hotend is still hot, catastrophic damage would result.
Finally, the best solution, and what any real hacker, maker, or engineer would immediately contemplate, is a dedicated fan controller. An Arduino and darlington or MOSFET transistor would be sufficient. Hotend thermistor voltage could be monitored, or a separate thermistor could be used at the hotend cooling fins. Any reading outside room-temperature would then trigger a fan startup and PID control sequence.
Ultimately though, all of these techniques for fan control are more complex, and therefore less reliable, than simply leaving the hotend cooling fan running continiously at all times. Further, any drop in cooling fan performance will correspond to an increased risk of severe jamming. Turning off such quiet, low-power fans should be of negligible concern compared with having a printer that works reliably. Other members of the community have been exactly right trying to explain that no control system for the hotend cooling fan is warranted.
More to the point, more details regarding the ACTUAL problem you want to solve (fan noise, insufficient hotend temperatures, noisy thermistor readings, etc) would be helpful. If you have to ask whether a series resistor would be able to reduce fan speed, you probably don’t have the expertise/methodology to know what the right solution is, let alone tinker with reducing hotend fan performance.
By the way, does anyone else see parallels with posts from “chriscalandro” about software fan control for the dual extruder? Perhaps these are related in some way…