WordPress site is hacked ………unknown posts

My word press blog , i don’t know how it got got hacked, the details are that unknown posts get published which i haven’t posted. Some one posts their posts successfully.

I always delete their posts , but its a headache doing it all the time. This is my site https://www.alphaability.com , i tried to change the admin password and username but to no avail.

Can anybody please tell me how to not let the hacker post their posts automatically , and solve this problem ?