Add closeable: false
to your lightGallery initialization. Here's their API describing that feature.
Working Codepen
<html>
<head>
<link rel="stylesheet" href="https://cdnjs.cloudflare.com/ajax/libs/lightgallery/1.10.0/css/lightgallery.css" integrity="sha512-I/g40Mx7U2Oepd3iHIpQRqdQGJ3vgdw0ix8LxGxX9zKv1MDizjD6dRcJ3PC1qpyfkj4rikVNcpBKcnmuJWUaTQ==" crossorigin="anonymous" />
<script src="https://cdnjs.cloudflare.com/ajax/libs/jquery/3.5.1/jquery.min.js" integrity="sha512-bLT0Qm9VnAYZDflyKcBaQ2gg0hSYNQrJ8RilYldYQ1FxQYoCLtUjuuRuZo+fjqhx/qtq/1itJ0C2ejDxltZVFg==" crossorigin="anonymous"></script>
<script src="https://cdnjs.cloudflare.com/ajax/libs/lightgallery/1.10.0/js/lightgallery.min.js"></script>
</head>
<div id="lightgallery">
<a href="https://via.placeholder.com/150">
<img src="https://via.placeholder.com/150" />
</a>
</div>
<script type="text/javascript">
$('#lightgallery').lightGallery({
closable: false
});
</script>
</html>
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