If you have had a transaction with them then no you can not stop them but you can report them.
If they are coming via ebay messages there is a report message at the top of the message.
Do you have the blocks to stop those that are blocked from being able to contact you?
Again this only works if you have not had a transaction with them.
To put that block in place hover your mouse over account in your my ebay page and from the drop down click on Site Preferences then go down the list till you see Buyer requirements and to the right of that click on show then click on edit. From there you can put in place the blocks you want, to stop those you want to block from purchasing from you.
Once a member has contacted you if you reply they can continue to contact you by clicking on reply to your message.
