By default all course participants have the right to read and write Wikis. Only those OpenOLAT users having created Wiki pages or registered Wiki owners are allowed to delete those Wiki pages. These pre-set rights can be adapted in the tab "Access" of your course element according to your needs.
Only registered Wiki owners are allowed to modify a Wiki menu.
If you do no longer wish to use a Wiki as collaborative tool but only as a kind of reference, (i.e. you do not want its content to be altered) just lock that Wiki's access in the tab "Access" of your course element in the section "Edit/create article."