I'd absolutely be pushing it back to the developer assuming it's a recent new build. From what I can work out:
- The developer incorrectly left it open for 18 months allowing it to become an informal thoroughfare
- Allowed their own contractors to use it therefore setting the wrong example to nearby residents
- Designed the layout in the first place that encourages people to cut through your garden
Assuming I've understood the above correctly I'd be escalating within the Developer to resolve it by installing a full width fence, additional thorny landscaping etc.
Doing nothing creates the risk it could become a permanent right of way, with you having to pick up the cost of the additional wear, possibly adding complications if you ever sell the property and having to keep the access open.
Our property is at the end of a long shared drive that would be used for access to the country park were it not for the railings that run the entire length. I wouldn't want there to be any gaps in the railing.