I think crotch sniffing is self-rewarding, regardless of negative attention so ignoring it would not diminish the behavior. Dogs like to smell, indentify, and explore with their noses, so every crotch, well...is a new adventure to a dog. (oh, that's a sentence you won't hear often!) It is more motivating than to avoid a correction, like you said he chooses when he will ignore a correction. If it's extra interesting & motivating, he'll do it.
I had a border collie who would stick his nose everywhere in a pushy manner-how obnoxious- and would push back against your hand when pushing him away. I would make a loud, verbal correction (ah-ah noise), frown & make eye contact and push into him with my legs, as if I was going to walk over him like a steam roller.
At first, he would just trip out of the way and maybe get a foot stepped on as he figured out what would happen if he didn't knock it off, but he soon began to scat farther and quicker away and stop soooner when I would start to correct him. Eventually he decided that he would rather not sniff anymore and get chased off.
This worked, I think, because I was consistently delivering an unpleasant experience everytime he wanted to sniff. He was a hard dog and bounced back from the correction with no problems. I consider obnoxios crotch sniffing to be a dominant thing, which he was. He would even hump our leg sometimes.
I preferred this type of correction over a collar correction because, as the pack leader, I was personally making a point to "get in his face", so to speak. (Don't really get in his face, or you could get bit.) I used my body physically to take over his space and make him back off while making a loud noise of dissapproval. The collar correction, I finds works after your pack structure is more recognized. I always left a drag lin eon himi though, as well, in case he was to become aggressive and challenge me.
Once he got into the right pack order though, he became very easy to train and would comply nicely (good pack driven dog). So, make sure you are practicing good pack leader ship and bonding exercises with your dog, since you just got him.
Watch your dog closely. When he moves close to you & begins making his way to your crotch, now's the time to correct. Try to determine when he begins thinking about sniffing or bumping you. That's when the correction should happen, don't wait for him to bump and be pushy and then correct. Correct for even thinking about being obnoxious.
If you notice him change his mind at the last minute, praise him.
You could also get one of those hand buzzers, like they use to trick someone when they shake your hand. Press it against that big nose when he sniffs. He won't like the sound or feel. But you have to have it on you everytime your around him until he's reliable, as well as anyone else in the room.
Hope this helps!
Alison Voore
Top Paw Training: serving Canyon Lake & New Braunfels, San Antonio to Austin.