Factory reset settings+ couldn’t unmount disk+ this disk is locked

Soo long shot, but the thing is I’m trying to factory reset my iMac since the password and Apple ID is forgotten. But all the issues started and I’m going to explain quickly all the errors and details I have until now: I can’t completely erase the disk from disk utility, I don’t know if it’s important to know but this disk has partitions called TALSPACE I can’t repair it because the error is live system repair is not supported . Also when I try to erase it the error is “couldn’t unmount the disk” when I try to reinstall OS X Mavericks the recovery HD it’s locked. I tried to force it with the command utildisk in terminal and is not successful. I’ve been reading the best option is to use an external hard drive. I don’t know if someone else can advice more about it