I'm not saying its pretty, but the only way I've been able to get rid of the instant upload pics is to logon to your Google + via PC, click on the pics icon on the left side, then click photos from phone, double click on one of the pics, then press view all, then there's a little tiny options selection there where you can delete pics one by one.
Seems like there must be a better way but I ran into this same issue and that's the only way I could ditch them. They need a delete option in the phone app!!