Gmail will not allow you to actually block somebody. What you can do is have it automatically delete the message from that person as soon as it comes in before you even see it.
Open the message from the person you wish to block
click on more actions
click on filter messages like this.
the persons email address should show up in the from section
click next step
click on delete it
then click on create filter
from then on any email from that person will automatically be sent to your trash folder as soon as it is received and will not show up in your inbox.
You do not need the actual emails. It would just have made it easier. You can click the create filter under more actions in your inbox and just type the persons email address in the from box in order to create the filter
then click next step
then click delete
then create rule