If you want to block the Invitations to the table you must click on the User Options button (indicated by the red arrows in the image below)

Then click on Online (indicated by the red arrow in the image below);
Then click on the Never button to the right of the word Accept table invitations (indicated by the pink arrow in the image below);
At this point you will no longer receive invitations to play.
When you want to re-enable the invitations to play, simply click on the button Always to the right of the word Accept invitations to the table (indicated by the blue arrow in the image below);