You will have to add maximum scale in script.
The following code should help you:
let source: String = "var meta = document.createElement('meta');" +
"meta.name="viewport";" +
"meta.content="width=device-width, initial-scale=1.0, maximum-scale=1.0, user-scalable=no";" +
"var head = document.getElementsByTagName('head')[0];" +
"head.appendChild(meta);"
let script: WKUserScript = WKUserScript(source: source, injectionTime: .atDocumentEnd, forMainFrameOnly: true)
let userContentController: WKUserContentController = WKUserContentController()
let conf = WKWebViewConfiguration()
conf.userContentController = userContentController
userContentController.addUserScript(script)
let webView = WKWebView(frame: CGRect.zero, configuration: conf)