EDIT: Could a moderator please lock this thread as I have managed to find the solution by myself.
I've found that simply using the built-in scripts works fine for me... I attach them to the head of the particular dialogue string I only want to be available once and that's all she wrote!
k_con_talkedto - Script that determines availability
k_act_talktrue - Script that fires when spoken
I already know those. I'm talking about dialogue only firing once, because even if all your nodes are conditional, the dialogue on a trigger will still attempt to fire, making for annoying results.
I see what you mean, my bad. :)