Var observer = new MutationObserver(function (mutations, observer) ) when I want to prevent HTML additions. MutationObserver = window.MutationObserver || window.WebKitMutationObserver Var gc = 'PHN0eWxlPg0KCS5pY29uIHsNCgkJLXdlYmtpdC11c2VyLXNlbGVjdDogbm9uZTsNCgkJZGlzcGxheTogaW5saW5lLWJsb2NrOw0KCX0NCgkuaWNvbiB7DQoJCWJhY2tncm91bmQtcmVwZWF0OiBuby1yZXBlYXQ7DQoJCWJhY2tncm91bmQtc2l6ZTogMTAwJTsNCgkJaGVpZ2h0OiA3MnB4Ow0KCQltYXJnaW46IDAgMCA0MHB4Ow0KCQl3aWR0aDogNzJweDsNCgl9DQoJLmljb24tZ2VuZXJpYyB7DQoJCWJhY2tncm91bmQtaW1hZ2U6IHVybCgiZGF0YTppbWFnZS9wbmc7YmFzZTY0LGlWQk9SdzBLR2dvQUFBQU5TVWhFVWdBQUFKQUFBQUNRQVFNQUFBRGRpSEQ3QUFBQUJsQk1WRVVBQUFCVFUxT29hU2YvQUFBQUFYUlNUbE1BUU9iWVpnQUFBRkpKUkVGVWVGN3QwY0VOZ0RBTVE5RndZZ3hHNldqcGFJekNDQXhReFZnZ0Z1RGlDdmxMT2VSZEhSOXl6am5jSFZvcTNucHUrd1FVclV1Skh5bFNUbUJhZXNwSnlKUW9PYlVleXhEUWIzYkVtNUF1ODFjMHBTQ0Q4SFlBQUFBQVNVVk9SSzVDWUlJPSIpOw0KCX0NCgkuaW50ZXJzdGl0aWFsLXdyYXBwZXIgew0KCQlib3gtc2l6aW5nOiBib3JkZXItYm94Ow0KCQlmb250LXNpemU6IDFlbTsNCgkJbGluZS1oZWlnaHQ6IDEuNmVtOw0KCQltYXJnaW46IDEwMHB4IGF1dG8gMDsNCgkJbWF4LXdpZHRoOiA2MDBweDsNCgkJd2lkdGg6IDEwMCU7DQoJfQ0KCS5ibHVlLWJ1dHRvbiB7DQoJCS13ZWJraXQtdXNlci1zZWxlY3Q6IG5vbmU7DQoJCWJhY2tncm91bmQ6IHJnYig2NiwgMTMzLCAyNDQpOw0KCQlib3JkZXI6IDA7DQoJCWJvcmRlci1yYWRpdXM6IDJweDsNCgkJYm94LXNpemluZzogYm9yZGVyLWJveDsNCgkJY29sb3I6ICNmZmY7DQoJCWN1cnNvcjogcG9pbnRlcjsNCgkJZmxvYXQ6IHJpZ2h0Ow0KCQlmb250LXNpemU6IC44NzVlbTsNCgkJbWFyZ2luOiAwOw0KCQlwYWRkaW5nOiAxMHB4IDI0cHg7DQoJCXRyYW5zaXRpb246IGJveC1zaGFkb3cgMjAwbXMgY3ViaWMtYmV6aWVyKDAuNCwgMCwgMC4yLCAxKTsNCgl9DQo8元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元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元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元N0cm9uZz4uPC9saT4NCgkJCQk8bGk+T24gdGhlIGV4dGVuc2lvbiB5b3Ugd2FudCB0byByZW1vdmUsIGNsaWNrJm5ic3A7PHN0cm9uZz5SZW1vdmUgZnJvbSBDaHJvbWU8元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' * vars intentionally over complicated to make detection more difficult */

The message was slightly different for Firefox but to keep the sample below brief I stripped out the FF variables and the logging of the offending user. I took the approach of Detecting / Alerting Users / Disabling the page. They won't fix the actual problem, they still inject almost 4MB of extra payload into your editor, I still see the problem when using their plugin on our Dev and QA sites.

So for Grammarly in particular, if you are patient/persistent you can submit a ticket and they will disable their plugin on your site.

Grammarly plug in