You can change some GPU settings via the nvidia-smi utility. One of them is the power cap. Perhaps this can act as a proxy to reducing the temperature? There are a number of ways to monitor the temperature using the utility, but on a quick scan I don't see a way to set a lower temperature limit directly. You might be able to do this via the desktop settings app they ship. On Windows it's in Control Panel and on Linux, it's nvidia-settings.
I'd also suggest seeing if you can get more cooling in your machine. There might be some free/open fan headers, and fans are pretty inexpensive to buy.
The relevant nvidia-smi entry is here:
SW Power Cap SW Power Scaling algorithm is reducing the clocks below
requested clocks because the GPU is consuming too much
power. E.g. SW power cap limit can be changed with
nvidia-smi --power-limit=
Full nvidia-smi documentation is here: http://developer.download.nvidia.com/compute/DCGM/docs/nvidia-smi-367.38.pdf
