My ex set up my computer and is a little possessive still. I want to make sure they can't monitor my computer usage, access files, etc. I wonder if there is a blanket setting in system preferences or something that I can use to decline/ reset all ssh permissions? I'm concerned they may have installed some kind of back door or something, with all good intentions at first for being able to help me out maybe, and I don't really know what I'm looking for. I'm running Kubuntu, the most updated version, and don't know any of the lingo or jargon. I just want to make sure my computer is closed to the outside.
If a person that you do not trust (no matter who this may be) has or has had administrator rights on the system, you may treat this as a compromised system.
A compromised system "does not belong to you", as in "you don't know what may be running on it or what changes has been made to it". You should reinstall the machine from scratch, making sure to first back up any personal documents beforehand. For maximum paranoia, you could avoid backing up personal binary executables (recompile these later after checking the source code) and personal scripts (read them first).
Ordinarily, one would not recommend backing up files from a compromised system, but instead to rebuild the system and then restore the most recent known untainted backup of personal files etc., but in this case this may not be possible or feasible.
When setting up the new system, you obviously would use new passwords, especially for root, and you would not set up accounts with
authorized_keys files (for SSH) with old keys.