hi George - despite some who may tell you that it's very unlikely - the answer to your basic question is Yes, it is *entirely* possible for someone with enough hacking knowledge and motivation, to do exactly what you're stating is being done.
as far as your internet provider potentially doing so, or any other company, for that matter, they most certainly *easily* COULD, but unless you're currently developing the next major computer program that will sell billions, are a top secret international spy, or are on the verge of discovering the cure for cancer, the chances of that happening are slim to ZERO...closer to minus zero, in fact...
therefore, the much more likely suspect would be someone who knows you personally....and has a reason to do so - those seeking revenge, a stalker, a co-worker, a bully (including relatives - especially siblings and ex's)...
yet, even if you had your computer tested and cleaned by Bill Gates himself, and even if you had the most up-to-date anti-hacking software installed, and had your wi fi connection and your computer password protected, if someone ever has access to your computer *itself* in the future - say, a sibling, "friend", or someone at school or work - they could potentially start the entire mess all over again...
therefore, to sum it up:
immediately check to make sure your wi fi connection is password protected - do not keep your password on your computer or anywhere else - make it something that you and only you can easily remember....
then, immediately change all of your passwords on all of your accounts - email, banking, etc...
then, have your pc looked at ASAP by a pro you can trust - a PRO - someone who is NOT a friend of yours, or a friend of a friend or relative - a pro who is solely in the business of computer repair and has a STORE - pay them for their work ABOVE the table - via credit/debit card...that way, you're 99.9% safe from that person themselves being involved in hacking into your computer...it's just not worth it to them....
after that, make sure your computer itself is password protected - meaning, even off-line, it'll greatly lessen the chances of anyone being able to access any info on it, nor add any malicious/hacking programs/software....
EDIT: lastly, since you think you know who's doing it, keep your computer away from this idiot at ALL TIMES, and get your *own* separate, wi fi account/connection, and make sure it's password protected. and, if your computer pro is able to track down any info on them, take the info to the police and file a report against them - immediately.