I always decline, then, I send a response to the person who invited me. I explain that this behavior is rude, (I give the benifit of the doubt to them). Then , most often, one of three things happen....
1. They don't respond and that is the end of it. (most often)
2. They respond with an explination about how they are new, and we talk about how to pug, then usually the matter is done (least often)
3. They are nasty about my (always polite) pm, then I post whatever nasty bile-filled garbage that they spewed at me into the general chat. I proceed to openly and politely confront their behavior for everyone to see and I refuse to let it go until they squelch me, or, they apologize.
In the third case, I have found that if it takes enough effort for people to be nasty, they usually quit doing it. Even if they don't want to. It has to be easy, when it's not, it's no fun. I don't bother reporting people, I just stand up for myself. When people see what is going on in the chat, they usually join in and back me up.