To answer your question, once you open skull, it is impossible to avoid yellow skulls because anyone can attack you which will give them the yellow skull.
In the example you described, it sounds like you are in red mode which isn't the right mode if you only want to open on 1 player.
This is red mode. Do NOT be in this mode:
Once you find the person you want to open on, click on closed fist mode, attack the person, and go back into Secure mode. This way you won't accidently open on additional players, but you can continue attacking the person you opened on. Keep in mind, this will not stop other players from opening yellow skull on you.
This is closed fist mode. This is the mode you want:
This is Secure Mode. The default mode when you log into an Open-PVP server.
TibiaGoals gives great explication of the modes- https://www.tibiagoals.com/2020/10/the-use-of-pvp-hand-modes-hand-mode-and.html
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