You can hide the button Pinned-Unpinned and the pin icon with CSS
Something like this should work:
.pinned-options {
display: none;
}
.topic-status .fa.fa-thumb-tack {
display: none;
}
If your users can’t see the pin icon they can’t click on to unpin a topic, the same thing if they can’t see the button.
(If you really really really want to do it)