Floating ideas about forced spending surely sounds desperate, but I doubt it will go through. There are simply too many practical obstacles to setting up a system like this, in a country like Greece.
It could even have the opposite effect: Some employers would start paying their employees a lower official salary, and the rest in cash. It is easy for restaurants, services & tourism industry, and already happens in many countries in Europe. The government would receive less even tax money, and people would have a lower electronic spending limit.
There are high earners who do not spend even 10% of their income, let︀ alone 30%. They would find workarounds, such as buying bogus online services from abroad, etc.,︁ or just leave the country altogether.