How to help people who may suffer from Paranoia?
I'm not active on this board, so please forgive me if my answer is not up to community standards.

I am not a psychologist, but I have some experience with mental illness (maybe too much.)

True paranoia (or Paranoid Personality Disorder/PPD) is much more pronounced than thinking that people can spy on you through your mobile. Whether it's true or not isn't relevant, really. In this case, there is a kernel of truth to their fears, which becomes exacerbated.*


  According to the DSM-5, persons with Paranoid Personality Disorder will have trouble operating with others in the workplace, educational or social settings (American Psychiatric Association, 2013). It is noted that people with PPD are more frequently unemployed or working more menial jobs than the general population (Mueser, Mischel, Adams, Harvey, McClure, Look, Leung, &amp; Siever, 2013).They tend to be solitary, self sufficient, and secretive, and will have difficulty making or maintaining intimate relationships or close friendships. (American Psychiatric Association, 2013). Persons with PPD may experience a conflict, in that they want intimate relationships and friendships, but do not have a level of trust which is an essential element of such relationships. 


If many of the other criteria aren't met, it's probably not paranoia. On the other hand, if your interactions are limited, you may not know about other criteria, so this might be paranoia.


  How to help people who may suffer from Paranoia?


It's very difficult, even for therapists, because challenging their belief systems makes the person very defensive. They do not tend to have much self insight, which makes therapy challenging. Clearly you are not in a therapeutic relationship with the user; I would try to steer them in the right direction security-wise, and then (gently and respectfully) disengage.


  So far, I have tried to tell those looking for help with their supposed hacker problems that they are likely not being attacked, and that it seems to me that they may be affected by paranoia. 


I would definitely avoid this, as this is giving a diagnosis, which you should not do (I believe it's unethical) over a brief interaction on the internet. I'm not surprised they counter by claiming that they are not paranoid and that their problems are real. To someone who's paranoid, their problems are real.

In case my answer isn't well explained,


  I'm not sure if bluntly saying "I think you may have a mental health problem" is beneficial though, and if it may do more harm than good. For this reason, I am asking this question, how exactly I should act and how to best help people.


Unless you are in a close personal relationship with someone, I think it's not in their best interests to tell them they may suffer from a mental illness, whether they do or not. First, you do not know. Second, they will most likely be defensive if not offended as well.

My reaction would be to be kind and gentle with the person, avoid overt confrontation ("That's not possible...") and let people close to them tell them they need professional help.

*Laptop cameras have been hacked, and worse.  

**I know people who believe Pizzagate was real. Te truth doesn't matter if you like conspiracy theories.

References

American Psychiatric Association. (2013). Diagnostics and Statistics Manual of Mental Disorders (DSM-5) Washington. DC: American Psychiatric Publishing.