I'm not familiar with the OM606 but there are several ways to do it. The easiest is to interrupt the vacuum signal to the EGR valve. An errant BB should do it. A more permanent solution is to put a cover plate between the EGR valve and whatever it attaches to. This takes care of leaks that the BB won't take care of.

In the 300D 2.5 there's a feature that cuts boost or the addition of fuel under boost if the ECU senses that the EGR is not functioning. Since there's no turbo on the OM606 I don't know if or how they'd penalize you. There's an awful lot of electronics in an OM606 so anything's possible. You might plug the hole and see for yourself if there's any difference. old south figured out how to disable the EGR system in his W140 300SD

I don't advocate that people go out and disable their emission control systems. I'm recapping what I've read in this and other forums In fact, if they have annual inspections including emissions tests in your area, you could be in for trouble if you disable the EGR system.

Sixto
91 300SE
87 300SDL
83 300SD