1. Is there geotextiles under the rocks?
2. Is there active irrigation?
The reason I ask is because the geotextiles will effect the herbicide performance. Irrigation will regulate how long certain products works.
I use Glyphoste in beds of customers who want repeat applications.......weekly maintenance. Fusilade II is used also to control some of the troublesome grassy weeds.
Glyphoste isn't going to stick around long as a pre or post emerge. It is a growth regulator will killing potential.
If there is not geotextile material under these rocks, then consider using Pendimethalin(Pre-M), Prodiamine(Barricade), Snapshot G(Treflan-Isoxaben) or Freehand. The first two chems mentioned is in liquid form and the later two is granular. I almost forgot clethodim for grassy weeds and broadleaf.
For my accounts that prefer non repeat efforts, I use the granular but be assured that grassy weed control will need to be done on two week intervals at least.
As far as using pramitol, spike, tebuthiuron,etc. These sterilants can become water mobile and move offsite to desirable areas. I wouldn't chance the outcome.