I'm pretty sure that people have tried every way possible to try it, but still he remained alive and kicking.
In Hellsing Ultimate: Episode 2, when Luke Valentine says he will prove that Alucard's immortality is a myth, Alucard replies that there is no such thing as immortality.
I did not really understand whether there are any limits to his so-called immortality, or maybe nobody knew whether there were any? Is there a way to permanently 'kill' Alucard?
After the final episode, when Alucard returns after consuming Schrodinger and killing every other soul in his consciousness, he is, as described by Schrodinger, everywhere and nowhere, both alive and dead but at the same time neither. He can exist wherever he wishes, and thus theoretically speaking, he could exist inter-dimensionally, as well physically and consciously, implying that no, he cannot ever be killed. He will exist forever.
This wiki has the following note about Alucard's immortality (may contain some spoilers):
Immortality: This ability is rather ambiguous, as Alucard, himself has stated that immortality is a myth. However, what it most likely refers to is the human souls inside Alucard; it has been speculated that, when damaged by blow that would have killed or incapacitated him such as exceedingly holy weapons, the damage is instead directed to his reservoir of souls. Because this ability does not really grant him true immortality, it can be considered as pseudo-immortality. However, after "Monster of God" Anderson destroyed the rest of Level 0, Alucard was still able to regenerate on par with him. (This was on par with Alucard before releasing Level 0). However, the "soul count" theory is just a fan theory, and has never been definitively proven. It was also specualted by Anderson that this was the reason for him not being able to kill Alucard. Alucard may have kept at least a few souls in his body to use for regeneration after releasing level 0.
Since there is no official answer to this, I would describe ALucard, as the wikipedia page does: Alucard is the closest thing to immortal. This is a perfect description that suits him well, until it is revealed is he really immortal, or not.
While people believe that Alucard is immortal, he says that he isn't, and we are later shown that this is because the only reason he is back from the dead is because he uses the enormous supply of souls he has absorbed throughout his unlife. However, at the end, he may have gained the ability to revive freely thanks to Schrodinger's powers.
By saying that he is not immortal, he may have meant that a being that is immortal doesn't mean it would be capable of handling such a fate and would wish to be killed. This side is greatly shown in the series where even Alucard himself wishes to be killed though by a person he sees as worthy.
Based on the current information, the only time Alucard was said to be mortal was when he released every soul inside him as an army, which made it impossible for him to use them to revive himself. However, this happened before he gained Schrodinger's powers. So unless more information is shown, the best guess is that Alucard can revive without limit now (like how Schrodinger did when he was shot 2 times but didn't die and reappeared fully healed, only to die when he willingly sacrificed himself in an effort to kill Alucard).
To know more please check the wiki. I am assuming all of this based on the information given and my own ideas. However, I believe this is the best description of Alucard's immortality as of yet.
Alucard sounds semi-immortal, unlike true immortals, who will never ever die, Alucard can indeed die, so long as he wishes it, or if he is killed by some kind of immortal slaying weapon (for example, supernatural weapons specifically designed to kill semi-immortal beings). The best way to defeat an immortal is by sealing them, basically separating them from the physical world, either by magic, or by placing in a burial chamber which they cannot escape from. However, while Alucard, an omnipresence being, can be sealed, it would require extensive planning.
Based on my knowledge of watching Hellsing Ultimate remember so many people try to kill him like Luke Valentine, polices, Walter and many other but the only one exceeded was the Major. Anyways he returned 30 years later. Just because Alucard is immortal doesn't mean these no way to kill him and he said " Only a human can kill me." I believe the only person can kill Alucard is his own master Sir Integra, this is my opinion but everyone has different opinions about is there ways to kill Alucard.
Except he already lost his identity when he first consumed schrodinger? He wasn't aware that he existed yet he still came back. Which leads me to believe Alucard is definitely immortal as long as he wishes to be. He can not be killed. On the higher being part when Anderson absorbed the helena nail that's almost like the power of God yet he won still. This leads me to believe Alucard is immortal.
