How to make a stamp or other signature immovable on pdf document (using adobe acrobat DC)?

There are two ways you can deal with this:

You can flatten your document (or the page the annotation is on) and convert the dynamic content to static PDF content. After that, you signature would look like any other static part of your document. This does not mean that with the right tools you cannot move the content anymore, but it would prevent somebody doing that by accident.

You can also apply document security settings that prevent modifications to the document. Once you do that, the document is locked, and nobody can (unless they break into the document) move anything around.

However, the better solution would be to not just use a signature stamp, but a true digital signature (which can show your "wet" signature - e.g. from a scan, or a signature capturing device). This way, the document can also be locked down (that's a setting you select when applying the signature), but more importantly, you can find out if the version of the document you have in front of you is actually the same version that you signed.