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Hi...

We had the same "problem" when we first bought our previous van...initially I thought it was a bl**dy stupid idea...and I did toy with swapping for a manual/toggle dump valve..

BUT..I ended up just using an "R" clip that I had, re-shaped slightly to slip around the stem of the valve, to keep it closed/up.

(...although a "H" or "Hairpin" clip would be better).

..if you look at the valve stem close, you'll probably see there is a groove at the correct level. This method worked well enough for the four years plus that we had the van..

Edit: Although having reread your post(properly this time ),it sounds like you may have a different vale to the one we had....

Ours was like this:

http://www.ebay.co.uk/itm/Truma-Electric-Dump-Valve-12mm-/180762616374?pt=UK_Campervan_Caravan_Accessories&hash=item2a164aa236

Is yours like this one shown on here?

http://www.truma.com/int/en/water-systems/accessories-water-system.php

..if it is....and turning the blue knob doesn't stop it from dumping, assuming that it isn't faulty(?),then maybe swapping for an "older" manual valve is the way to go then...?

Re: Clothes peg(lucky or otherwise )

Judging by scotfree's(the OP) reference, to a "blue  lozenge", it doesn't sounds as if his valve is of the type that will work on(...the drop down,"plunger" type).

His sounds like the one shown in the second link I posted above....

http://www.truma.com/int/en/water-systems/accessories-water-system.php

and as he: " repeatedly turned the blue lozenge shaped lever on the top of the black valve unit"   and it had no affect ,then one can probably assume either,

 a)The "auto-dump" in designed to over ride any manual intervention

 or

b) It should be able to be "over-ridden" but it's faulty....  

If it is okay..and just isn't meant to be "manually" held closed , then I'd probably look at swapping it for the simple, yellow toggle type...

I know the intension of these valves is to stop frost damaging the boiler but they do(or ours did!)seem to "dump" even when they was only a slight  "chill" in the air!..and  I wouldn't want to need to keep  the heating on, just to keep a valve closed..