Make it an option in admin cp.
Add a tag to the links in the signature - "nofollow". To all links
in signatures (except for internal links to other parts (page in the forum) have been tagged "nofollow". And just to be able to added a tag in this setting (if it is also about other search engines from which the other instead of nofollow tag.
Another similar setting in the admin cp to do for all messages (posts) written by users
for
example: picture
View attachment 1423
in admin cp.