Surreptitious posting

Someone, having no user account, has been leaving posts on my site that I never asked for. The posts always appear with a date stamp that has the same month and day in which they appear, but dated a year ago. One just appeared today (March 9, 2015), dated “March 9, 2014”

I suspect the only way to put a stop to this is to change the password WordPress uses to communicate with its database. How can I do that without risking incomplete propagation of the new password throughout the site?