How to Block a Caller on Skype
"Not all Skype calls come from people to whom you want to talk. If you receive calls from nuisance callers, you may want to block them from calling you in the future. You have two options when blocking callers. Skype allows you to block calls from anyone not currently on your contact list, which is the preferred option if you receive nuisance calls from a multitude of numbers. However, you may also block a single contact, which is preferred if you need to keep an open channel for non-contacts."