Cent delete one specific text message on iphone 5?

I have a 6GB conversation with my boyfriend, but its taking up way too much space. Every time i try to delete the conversation, it disappears but it doesn't allow me to send or receive texts from anyone. I have to restart my phone and the conversation is back. That is the only way i can text people again.

How do i permanently get rid of our conversation do i can get more space on my phone?

Try it again but wait like a half hour or maybe an hour, it might be taking a really long time to delete all these messages.