PlantronicsHub and a major memory leak
Though I regularly search for answers here, this is my first post. Please, forgive if I do not match this forum's etiquette.

I fully empathize with your problem. SpokesUpdateService is relentless. For me, it seemed to launch around the same time each day. I knew it was crashing my Late-2011 MBP with Sierra 10.12.6, but today's EtreCheck simple (free) "diagnosis" show this damn SpokesUpdateService caused 113 crashes on November 3rd for me alone. (No wonder my system died and refused to restart for hours!) 

Realistically, Plantronics rarely updates the firmware or software for my headset. I can check for updates when I choose, not hundreds of times daily with SpokesUpdateService.

Here is my simple brute-force solution:

Trash the following three files: 


com.PlantronicsUpdateService.plist 
Headset Updater (Version 1.1.0 (2015), a Plantronics application
Plantronics Hub (Version 3.11.52215.23527 (2018) another Plantronics Application. 


You might also get away with only trashing com.PlantronicsUpdateService.plist or editing it (see below).

I deleted these files because com.PlantronicsUpdateService.plist seems to call SpokesUpdateService:



For good measure, I also trashed the applications "Headset Updater" and "Plantronics Hub." I still have all three in the Trash in case I need them later. Simply putting these three files in the Trash has stopped SpokesUpdateService. I most likely can permanently delete all three and simply re-install the Headset Updater and/or Plantronics Hub if I need/want them.

I have not programmed in decades, but you might try editing com.PlantronicsUpdateService.plist by deleting "KeepAlive". I say that based on commands at "A launchd Tutorial", http://www.launchd.info (last visited Nov. 9, 2018).