Samsung Galaxy Ace 2.
Let's say I have 3 desktops, I can scroll from 1 to 2, from 2 to 3 and... from 3 to 1. The last one is undesired by me, so how I can disable this effect?
It is likely that the option does not exist on your stock phone launcher (can't confirm that, I have a different phone), but if it does, it should have the keywords "looping", "wrap around", "circular scrolling" in the settings.
I would recommend installing one of the following 3rd party launcher instead, which do not wrap around by default, and provide other useful functionality:
Go Launcher Ex appears to be in active development though so you'll probably have better support going with it. There are other launchers as well but these 3 are the ones I have used before.
Just use your fingers in the main page. Place one finger on the top of the screen and the other on te bottom. Then, diagonally, slide them untill the middle, joining them. That should expand all the pages you havr. After that select the one ypu want by long pressing it and slide it to the bim to remove it. Good luck!