I had a scan through the manaul, and it appears you can't do this from the front panel of the Di.
However the MIDI implementation chart on page 127 does mention that Hold 1(CC#64) can be set to receive or not but you need to use the Juno's editor program.
So perhaps there's a setting using the editor program that allows you to set hold to be ignored.
Do any of the preset performances have any tones that don't sustain if the pedal is 'held', that would certainly point the way to prove it's possible?
Thanks ever so much again. I've just managed to do it. I had a look in the manual for the editor, had a search for "hold" and it told me that the setting "RCV-Hold 1" in Control SW can be turned off to stop a patch being held down. This is great because I'd been trying to do this with my last keyboard for ages.