Do not block them (since there are probably also other victims), report them either for PM'ing or any other offense that corresponds to what kind of message they sent you. Click on the username, than "report" and than select the reason you judge appropriate. The admin team will than look into it. If multiple people report the same user they know something is wrong and can take action.
Just stop talking. When you stop responding, they usually stop as well. If they persist you can always block them, very easy to do and they'll no longer be able to contact you. If you think they're being offensive or harassing you, you can always report them.
There's also an option in your settings to only allow people you follow to be able to message you.
I've only just joined a little more than a week ago and unfortunately I've had gotten some creepy messages from 5 or 6 people. Some of the profiles "aren't active anymore," but I changed my settings to private and block anyone that won't leave me alone. If it keeps up then I might have to delete my account.
Many times. Today I've had 5 different people messaging me about how pretty I was and one of them wanted to sex talk. I just say thank you and wait till they'll start a normal conversation. The suxual ones, I just tell them I'm not into that and if they push I block them.