How can I reduce the heat output of this heating element?
First, you will want to check the wiring and fuses in your current smoker to make sure it can safely handle the 1500w upgrade.

Assuming that your wiring and fuses can handle the 1500w element, keep in mind how the system works in the first place.  The heating element is controlled by the thermal measurement unit.  A more powerful element would still turn off once the set temperature at the measurement unit is reached.  The difference would be how fast it reaches the temperature and more hot spots closer to the element.

For the "variable" options you can alter the voltage or the current running to the heater. One issue to be aware of is that you would need to keep the power supply to the smoker's control board separated and unaltered.

Altering the voltage:

There is a power supply called a variac or variable transformer.



Pros:


Since this plugs into the a home receptacle and has a standard receptacle coming out of it, it would be safer from a wire splicing perspective.  Keep in mind, you'd need to acquire a variac that can the wattage.
You will look and feel like a cold war submarine commander when adjusting the voltage on this kind of equipment so this is a high testosterone solution.
There's conversational value here for parties.


Cons:


A potentially fiery death.  Playing with high wattage hacks requires strict attention to detail and close supervision of the device.
Cost.  The price ranges for these seem to be $180-$400ish in the 2000W range.  This could significantly increase the overall cost of your smoker setup.
The control board of the smoker can certainly cope with some range of voltage under the usual 120V, but it may not operate at the lower voltage 
Even after everything is working you probably don't want to invite your fire insurance provider over for dinner if you like your current insurance rates.


Altering the current (by adding resistance):

You can add a variable resistor.  Just like the above solution you'd need a higher wattage rated device.



Pros:


It would indeed adjustably control the amount of current flowing through your element.


Cons:


A potentially fiery death.
This resistor would be handling some serious current and it doesn't have any housing as-is.  You wouldn't want your cat to brush against this.
This solution require wire splicing and would be inherently less safe for anyone not very familiar with electric systems.
Cost.  This is in the $350ish range
This could heat up depending on the setting you choose.


Summary:

Both of these solutions are costly compared to most smoker's retail value.  These higher power devices require alloys that are expensive to produce.

The variac would be my preferred cooler and safer solution out of the two, but keep in mind that the smoker control board wants plain old 120 line voltage.  It will not operate very far out of the 120 volt range and, even worse, could malfunction and not turn off the element.

Regardless of any solution,If you do proceed you will be hacking a 1000+ Watt device please be very careful.