I don't think that the spear has anything to do with Alucard sealing his powers. Besides, unless I'm mistaken, that spear was an item in NitM. Anyway, why would someone seal their own powers in something. It would make more sense to put seal on themself that prevents them from using their power. This way the power doesn't fall into the wrong hands and if the person needs to, he/she can unseal their powers.
This is also the only way I can think of that Alucard could have regained his powers so quickly in DoS. Then again, I don't think that he completely sealed them away because he had some power in AoS and DoS. How else would he have been able to get around with so many monsters around and not get killed? Besides, in AoS, he use the soul steal spell in the opening scene.
The spear just may be something that Alucard had and gave away. Unfortunately, there is clear evidence that makes to case for one or the other. So all we can do is speculate.