You could clip it open, but you risk going too far and having flat beer that won't have any head on it, even when served on nitro at 30 PSI. Here's what I would do... Set the regulator to 30 PSI and leave the gas shut off to the keg. Purge the keg about once per hour and each time, try to pour some beer and if it still foams excessively, you're still not there yet.
Each purge cycle should go like this:
1) Purge the keg
2) Turn on the gas (you need to test it at serving pressure, I.e., 30 PSI)
3) See how it pours
4) If it still foams too much, turn off the gas and purge again, then wait an hour and do it all over. You'll know you're done when you can get a decent pour out of it. This will waste a bit of nitro, but if the keg is full, it won't be that bad.
Keep in mind, that even when the beergas setup is working properly, and the carb level is correct, you'll still get a lot of foam. I believe a proper Guinness pour requires two pours, the first of which is almost all foam, then a 100 second pause, followed by the finishing pour. This is consistent with how my nitro setup pours, as the glass is almost all foam after the first pour. Also keep in mind that nitro beers are about as close to flat as you can get without them actually being flat, so while you'll be drinking beer that has a nice creamy head from the nitro, the beer will essentially be flat like a Guinness. Are your guests going to be okay with that?