Once you get the guild invite hit "n" to bring up the notifications menu. When you click the "x" to decline the invitation you get three options: Decline, Report, and Ignore. I report, then ignore, then decline.
EDIT: As far as blocking blind guild invites I don't think you can. I looked through the UI but didn't see it. I checked the forums and all I found was "you can't." I used /feedback in game to ask for the function but I am just one person. It may take quite a few requests be it becomes a priority.
agreed need something else to block these, reporting them does nothing they hop on to another character and repeat the process, it started for me 2 days ago and today its been constant declining of fake guild invites, i hate RMT and its ruining my enjoyment of the game
Have not got spam invites yet. It is better to keep reporting them, then pretend that they are not there.
Like the old saying:
"One can not cure cancer by saying it is not there"
Also, ZOS is working on the problem and they have ban several thousands accounts already. If the players work on this problem with ZOS, we can beat them! Let us at least try to make this a better game and fan based. We can not give up hope for that goal and always work towards it.
I was surprised to find that there's no setting to block unsolicited guild, party, and friend invitations. I thought the need for that was a no-brainer.
(edited for spelling)
Edited by Sue_D_Nim on April 12, 2014 6:43PM
"If danger doesn't find her, she'll seek it out and invite it home to dinner." ~ Prince Naemon
"Such despair! Richer than a cheese sorbet!" ~ Sheogorath