Inactive Tutor answered 03/26/19
Ciao! Great question! Are you familiar with Italian commands (gli imperativi)? The negative "tu" command for is expressed by non plus the infinitive. For example, if I tell you not to eat the ice cream (not sure why I would do that, but oh well!) I would say "NON MANGIARE IL GELATO!" as that is the negative tu command.
Now, let's look at your sample. If it were not a reflexive verb, we would say NON FERMARE MAI, however, it is a reflexive verb being used for the negative tu command, therefore, the "ti" becomes attached to the end. We cannot put it in the beginning because it is not followed by a conjugated verb leaving us with NON FERMARTI MAI.
I hope I did not make it too complicated. PLease feel free to follow up!