Hi Guido, if you think there's a more apropriate word to translate this request, you may edit and accept or reject the translation and submit your own.
I would like to know more context, like the interface page that it is to be used on. What is it that has to be locked? *sluit* is more like close/fechar [not necessarily à chave]. You can *sluit* a door or a tab or your browser. But it wouldn't do for bloquear [uma página/uma pessoa, for example]. And that is the Portuguese translator's interpretation. You would have to use something 'stronger'. Like *blokkeer*, or *vassit* if you prefer a purism. Sorry to be such a nuisance.