I'm not entirely sure what your questions are - let me try to at least comment:
1. No, I do not believe that you can be prevented from astral projection by anyone but yourself and your own fears/attitudes/beliefs. The ability to leave one's body in a dream or awake and go adventuring in bodiless form is something we all have, and no one can stop you from using it but yourself. However, you might be unpracticed at doing it consciously, so that you aren't "good at it" yet; or you might be preventing yourself from success by your fears/attitudes/beliefs - you'll need to address them and perhaps change them before you'll succeed.
2. Hearing other voices could be anything from hypnagogia (images and sounds that appear in our brains as we're falling asleep; akin to dreams) to telepathy (hearing others' thoughts) to remote "viewing" (hearing others' conversations without actually being physically present for them) to, very simply, "tuning into" another reality. Or more!
3. In the case of having trouble succeeding at astral projection, the solution is probably practice (meditation, observing the waking-to-sleep transition, energy work, visualization, exerting the will in the direction of your intent) and belief work (re-aligning your beliefs so that you are not getting in your own way!). In the case of voices, no solution is needed because there is not necessarily a problem.
4. If you truly believe you are under psychic attack, there are spiritual/magical measures you can take. A very simply spell is to visualize yourself surrounded by white light (or any color that speaks to you of safety), such that no ill intent can cross into that light to harm you. A more complicated one, as I understand it, is the Lesser Banishing Ritual of the Pentagram - it is part of ceremonial magic traditions, which I am not so well versed in. If you are religious, and prayer is part of your religious tradition, then pray. Contact whatever Deity, spirit guide or totem animal you believe in, and invoke their protection and strength.
Myself, I do not feel concerned with psychic attack. I do not believe any psychic attack against me can succeed without my cooperation. So my own solution would be to make sure I'm not cooperating! Pessimistic attitudes, fear, feelings of helplessness - these are themselves causes of ill fortune, unsafety, and helplessness. So I address and correct those, rather than look for some external source of protection or healing, in these cases.
If you can get ahold of a book by Rick Stack called "Adventures Out of the Body," I recommend it. He is a strong proponent of the sort of belief work I mention above.