How to prevent "com.apple.siriactionsd" folder storing millions of spurious items?

My Temporary folder is zero bytes. You may have a cache file that is damaged and is causing the problem. To test that theory do the following: boot into Safe Mode according to How to use safe mode on your Mac and delete the contents of the Temporary folder. Reboot normally, run for a while and test to see if the problem persists..

NOTE 1: Safe Mode boot can take up to 3 - 5 minutes as it's doing the following; 

• Verifies your startup disk and attempts to repair directory issues, if needed

• Loads only required kernel extensions (prevents 3rd party kernel/extensions from loading)

• Prevents Startup Items and Login Items from opening automatically

• Disables user-installed fonts 

• Deletes font caches, kernel cache, and other system cache files

NOTE 2: if you have a wireless keyboard with rechargeable batteries connect it with its charging cable before booting into Safe Mode. This makes it act as a wired keyboard as will insure a successful boot into Safe Mode.