Zoom creep is fairly common in some ultra zoom lenses like your Sony. My Nikon 18-200 suffers from it if the lens is pointing almost straight down. Like the Sony, Nikon added a zoom lock to hold it in place when not in use. This is of limited use since it locks it at the minimum focal length only.

The problem should mainly manifest itself when the lens is pointing down. If it is so loose that it creeps when the lens is almost horizontal, then that sounds like a real defect. Is this something that has developed over time, or was the lens this way when bought? If it is a new lens, you might see about a replacement. But it is not an uncommon trait with the 18-250 and other zoom lenses, so depending on the extent of the problem it may not warrant the cost of a repair, if at your expense.

Some people have had success in reducing creep by using rubber or elastic bands around the lens. Or if you're shooting for a time at one focal length, a small bit of gaffer tape should hold it in place.