In Kamisama Hajimemasita, Kissing means to make contract with familiar. In Akura Ou Arc(manga), to enter into the flame, where Akura Ou's body lies, Tomoe turns into youkai form by just kissing Nanami. Nanami was unaware of that the kissing breaks the contract. Kissing works like off an on button. Kiss, make contract and kiss, break contract.
If only kissing can make and break contract, then Nanami should have realized about this. They had kissed many times. I don't think they were counting, so that the contract remains intact. And breaking contract changes the appearance of Tomoe to his youkai form. Nanami should have seen this change.
I don't remember the episode, most probably after returning from Underworld for the first time, Tomoe says he wants to kiss to make the contract from his heart and he already had made contract, in that case, they break the contract.
Is there anything else beside kissing, while making and breaking the contract. Like feeling or wish of any one of the member of the contract.