I presume the CMA doesn't have worldwide jurisdiction (I mean, even hypothetically, under UK law). However, if a merger went ahead against its wishes, I presume it could easily block the Figma portion of Adobe operating in the UK.
I think the only way to do it would be to entirely pull Adobe out of the UK market. This would be extremely difficult for many reasons, not only that Adobe has many multi year enterprise license agreements with many UK companies which would incur enormous penalties for severing them, plus the UK is probably still a quite large market for Adobe.
I'm not even sure if that's possible though, as they could still get a judgement in the UK and then take that to other jurisdictions to get payment for fines and other breaches.