Last week, a neighbour kissed my baby on his lips. I was so surprised, I didn't even respond. I think it's unhygienic and disgusting. Am I overreacting? How can I be polite, but make sure this doesn't happen again?

Definitely not over reacting.. I think it is highly inappropriate. Just like how one should never just carry a child away (like how many love to do) without seeking the parent's approval. I think if your neighbour tries to do that again, subtly shift your baby away when he/she start leaning in. Then politely explain that you would prefer if he/she could just pat your child on the head (or some other gesture) to show his/her affection. Then perhaps ask your baby to blow a flying kiss to say good bye.