Short answer: Yes, they are.
Long answer: Unfortunately, we can't prevent users from doing any changes to their signature in their Gmail Settings.
You can let users sending signature change requests via their dedicated page. Please check out more information in this article.
These requests can be automatically approved.
But be careful! If you use the credit system, you'll be charged in the case that the signature is rolled out automatically. There's also a possibility that the user might do inappropriate or otherwise harmful changes, so unless you trust your users completely, we suggest keeping the option unchecked and review the requested changes manually.