I agree with the previous post about diagnosing what's causing the symptoms before trying to treat it. It could be a number of ailments.
My flock had infectious coryza go through it a couple years ago. It got into my flock (which had been clean for over 5 years) because someone found a stray hen and thought it was mine, and tossed it into my barn! Some of the birds died of it, while others had mild symptoms and recovered on their own. But they are permanent carriers.
Because, unfortunately, I was too sentimental to cull the survivors(they are ornamental bantams), I chose to run a "closed barn" instead, meaning I will not sell or swap away any of the birds in that flock, and maintain strict biosecurity and don't allow anyone into the barn except with special "barn" clothes and boots, etc. I had to put the entire flock on erythromycin (Gallimycin) as a precaution against subsequent outbreaks. I haven't had any problems since, though, and it's been over a year. But it will always be a closed flock.
You have to get crazy-strict to keep infectious diseases under control.
As for taking care of the "boogers" themselves. A couple of my birds recovered but, typical of infectious coryza, continued to get outbursts of the disease from time to time. If I didn't removed the boogers, eventually they would back up into the birds' sinuses and cause the entire head to swell. Really, the "boogers" are pus that drains from the sinuses and hardens to a crust on the outside where it contacts the dry outside air.
I took sterilized (in bleach, then rinsed) splinter tweezers and VERY CAREFULLY (bird wrapped gently in a towel to immobilize it) removed the plug - it usually came out in one piece... but I also cleaned out any remaining "booger" so that the nostrils and nasal passages were clear. Then I dabbed a little triple-antibiotic ointment just inside the nostril (not clogging the nasal passages).
