Theseus is a nulla tempora comptus acu, whose parts can be understood as follows:
Comptus is roughly "tied together".
Acu is a normal instrumental ablative, "with a pin".
Tempora is an accusative of respect, also known as a Greek accusative (accusativus respectus/Graecus).
It has the same meaning as the ablative of respect and is predominantly used in poetry and of body parts.
What we have here is a poem about a Greek myth and a body part is involved, so the accusative is not a surprising choice.
A nulla is an agent, "by no woman".
Knowledge of the poetic metres helps here, as it makes it obvious that the A in nulla is long and therefore it is the feminine singular ablative.
Without that hint we may have been led to believe that it went with tempora.
An alternative worth considering is that nulla is an attribute for the feminine acu.
If you read them together, then you have to explain the preposition a.
The phrase a nulla acu can no longer be instrumental (it should be a plain ablative or perhaps come with cum) and it cannot be taken as an agent either (agents like nulla acus would go without a preposition but the agent nulla does use an a).
This line of thought does not seem to lead to any tenable reading, so I would take nulla and acu to be unrelated, unless there is context beyond those lines that strongly suggests otherwise.
To turn this into a sensible construction in English, let us start with just the participle and the accusative of respect:
[Is est] tempora comptus.
"[He has been] tied together with respect to his temples."
I supplied the is est to make it clearer how this is rephrased to better English.
The "with respect to" is a clunky attempt to mimic a Latin structure that isn't there in English.
We should, instead, say something like:
Is est tempora comptus.
"[The hair of] his temples has been tied together."
The hair is understood from context.
You can choose to leave it out, but I find it more idiomatic to leave it in.
Now we can add the other two elements:
Is est a nulla tempora comptus acu.
"[The hair of] his temples has not been tied together by a woman with a pin."
It is my strong opinion that it is best to form and understand this mock-up sentence first before trying to put the ideas into the whole original sentence.
With this new understanding we can put it in the context:
Minoida Theseus / abstulit, a nulla tempora comptus acu.
"Theseus carried away the daughter of Minos, never having had his hair pinned at his temples by a woman."
There are many possible ways to phrase this, but the message is the same:
Theseus had never allowed a woman have a say on his looks, but this was probably about to change.