From the text of the Akallabêth, it is clear that following the creation of Númenor, the "Ban of the Valar" forbade the Edain from sailing anywhere near Aman, or even Tol Eressëa. And the earlier portions of The Silmarillion make it clear that, for quite some time, the rebellious Ñoldor were likewise forbidden from returning to Aman. However, the question of whether men were ever permitted to approach Aman is far less obvious.
When Eärendil arrives in Aman to ask for the help of the Valar, most of them aren't very happy. Mandos makes a thinly veiled suggestion that the Valar should kill him rather than allow a "mortal man" to set foot in the Undying Lands. Of course, Eärendil is actually half Elf and half human, and the Ñoldorin Elf part of his heritage unquestionably denies him access to the Undying Lands, but it seems significant that Mandos specifically referred to him as a "mortal man" before the issue of his Ñoldorin ancestry is raised.
I don't recall reading anything to suggest that men were always banned from Aman, but Mandos' words seem to imply that a ban was already in place; if I'm not mistaken, the first reference to such a law occurs in the Akallabêth, after the creation of Númenor. Did I miss something here? Was there always a law forbidding humans from traveling to the Undying Lands?
Note: While the title of the question "Since when did Eru forbid men from going to the undying land?" is similar to the intent of this question, the actual content of that question is wildly different; despite the title, it actually asks about some sort of hostility shown by the Valar towards men. The question and the answers to it don't address the issues I raise here, and as far as content is concerned, there is almost no similarity between the two. I couldn't get the kind of answer I'm looking for from that question without changing it so much that it had no relation to the OP's intentions in asking it.