The original dump valve - on the side of the turbo - should be blocked off when you fit an aftermarket one. This can be done by taking the cover off and fitting a metal ring to keep the valve firmly shut. You can also attach a bit of hose with a bung in the end to it but the first option would more desireable.
If you're running high boost you may also run both dump valves at the same time by adding a T-piece to the equation.
The original dump valve is a recirculating one and is integral to the turbo unit (under the inlet hose). When you fit the aftermarket one (between the intercooler and the inlet plenum) you should block the original off using the methods mentioned above.
You can also run the two together, the idea of a dump valve is to release the pressure between the shut throttle plate and the turbo unit.
is it easy done to block it off the 1st way you mentioned?
do you have to buy a proper part or anything?
You undo the 4 allen key bolts that hold the plate on to the turbo, insert a ring that fits onto the existing valve face (basically wedges it shut), then reassemble. You'll need a ring of the right size, I've got 2 lying around somewhere but there's been a few posts about the size of these things in the past which should turn up after a quick search