It seems to be Hard Wired in the LR program files and right now can't be moved. Big flaw IMHO. Everyone that is looking to do this same thing needs to contact Adobe and request this option be included.
The only thing you can do is Limit the Cache size to say 1 or 2GB.
In my experience the cache size is ignored by LR.
To prevent LR from caching I resorted to renaming dynamiclinkmediaserver.exe that is responsible for it in:
C:\Program Files\Adobe\Adobe Lightroom\Helpers\DynamicLinkMediaServer\dynamiclinkmediaserver\1.0
I just appended it with "old", i.e olddynamiclinkmediaserver.exe
When you run an import LR can't find the file hence it doesn't run and it doesn't cache. The downside is that you can't preview any videos, but it stops the cluttering up of the HD.
Any problems, just rename it back.
In the mean time, perhaps add your support here: Lightroom video cache import bug causing large amounts of HD space to be used up, and I'm not importing videos!
WOW, I was not expecting such a big flaw in LR.
I will try to reach out to Adobe and ask inclusion on next update of Lightroom 6.
Thanks for the reply Andrew.
Unfortunately not being able to preview videos renders the usage of LR on my workflow useless. So I am afraid this is not an option for me.
Could anybody tell me how to contact Adobe directly.
It seems that the only option on my membership plan is access to this Forum.
Is there a number I can call or is it possible to chat with someone from Adobe?!
For everyone who struggles with this bug, you can use this as a workaround:
Just change the directory from the Downloads folder to:
"C:\Users\YOURUSERNAME\AppData\Local\Adobe\Lightroom\Caches\Video\Media Cache Files\"
Also, change the timeframe in the .bat file from -30 to -0, that will delete all the files in the folder, not just the files older than 30 days.