How to delete iPhone WhatsApp group chat history quicker?

I'm trying to delete this group chat conversation from my iPhone 4S which has been going on for more than 4 months. When I try to delete the conversation, every 10 minutes my phone crashes and I find out nothing has been deleted. There have been more than 40K messages over the 4 months. I would just like to find any alternative way in which I can delete this chat history and leave the group quicker other than deleting the application because I have to buy the app again for 59p

Actually you can dowload the app again for free because apple appstore allows you to buy it for free it will show a message saying you don't have to pay for it when you download it