Make Scout Space Marine's stun grenade an individual upgrade ie. player needs to upgrade each of his scouts to have stun grenades one by one.
Devs can then balance this gradually by adjusting the cost for that upgrade following feedback.
Making it an individual upgrade on a unit by unit basis slows down the spam but not entirely removing the option. So if someone really wants to build a chain CC combo he still can but the process is made more tedious and cost rebalanced. Hopefully making other units more attractive.