They could be extracted with great Allomancy and it was intentional.
In later books (if I'm not wrong, in The Hero of Ages) it said that the Lord Ruler did that in order to keep the Inquisitors loyal to him. He designed them with Hemalurgy in a way that would make them vulnerable to his pulls/pushes. This way, they would be forever live forever with the notion that their power came from him, and that he could kill them outright if they rebelled. I'm unable to find the exact passage, but maybe someone else can.
This is shown again when
Elend fights a steel Inquisitor in the Hero of Ages, and he discovers that the inquisitor had protected his central spike to avoid someone pulling on it (physically, at least).
This can be seen as Inquisitors trying to prevent the flaw they've been created with,
since at the time, with the Lord Ruler being dead, they weren't exactly "in check" anymore.
Generally, it's unlikely that anyone could push or pull on the spikes due to the nature of Allomancy. It is said that Allomantic powers have decreased over time (due to the power being passed down generation after generation, and "watered" down in the lineages) and we don't know if in the "golden era" of Allomancy it was possible.
The Lord Ruler was both an incredible Allomancer, and had a nice set of tricks up in his sleeve, so he could.