I was wondering if it was ok to downvote or upvote all the threads made by one person for personal reasons regardless of the threads content ?
Your question is interesting because this is the typical consequence of the reputation system that exists on Stack Exchange ecosystem.
This system of rewards provides a "gamification" layer that is in my opinion a very powerful approach to hook the user interest and stimulate its involvement to the community. But as every systems offered to a community, it can motivate users to use these tools in the wrong way.
I understand that the temptation to "help" or "penalize" an user for personal reason may be important, i already faced such kind of situation when i invited a friend to join me in the Stackoverflow community. After a quick look at the situation, i realised that it was completely counterproductive to influence his reputation (in a good or bad direction). The first reputation points are so important for the learning process, an user needs to get involved in the community step by step, privilege by privilege.
Another power of this ecosystem is that the real value of the platform resides in the sharing of knowledge and the relationships between users are almost inexistant. The exchanges between users have a temporary lifetime (chat session, investigation to solve a problem,...). There is absolutely no interest to influence the reputation of an user for personal reason because at the end the only thing that matters in this ecosystem is to find answers to a given question, learn from the others and share knowledge.
To answer your question, you can influence the reputation of an user but it will be a waste of time for both of you. Focus on the opened questions, try to help the others, take part to investigations, ask questions... There are enough social networks for personal interactions.
Hope that this answer will help you.