how do i make it so my messages do not remain with my phone after i restore and backup from a previous backup on

im trying to figure out how to backup your iphone to your computer without backing up the stored messages, so when you restore your phone and back it up to the previous backup file, the messages do not come with it. Ive been having trouble with my phone not deleting messages, whenever i delete a message thread, the app freezes, then when i click the home button and close all running apps and re-open imessage, the messages are all wiped out, so when i restart my phone, the messages all appear again, even the ones i wanted to be deleted, leaving me unable to free up any sort of space on my phone due to this obvious bug. SOS