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?

No, you are not overreacting. Yes, it is not only unsanitary and disgusting, it is also inappropriate, dangerous and unhealthy. Babies do not have strong enough immune systems to defend themselves against a number of viruses, so they easily catch viruses from adults. Be firm and tell him that you appreciate that he finds your baby cute, but you do not want him touching or kissing your baby. Sure, he may be offended, but it's for the good of your child. The next time he tries that, yell out "No!".