I haven't taken pins/plugs from an ECU harness but I have pulled pins from distributor connectors, O2 connectors, and the like. Perhaps there's some commonality. For the ECU harness (ODBI) the ends of the connector flip up (someone showed me this). The pins push out from the front to the back. For the distributor connector, O2 sensor, etc. that I pushed pins from I used a set of jewelers screwdrivers. Don't let the name fool you, they weren't expensive. They are just tiny flat blade screwdrivers with a pivot on top. The set was probably something like $8 or so.
Anyway, that's the tool I used to push pins out. The blades are very tiny and fit into place to push the plastic tab and/or pin out. I also heard you can use a "scratch awl". This is a woodworking tool that looks like an ice pick with a round ball like handle. Heck an ice pick might work too.