Sorry I have been researching and looking. And can't find anything that allows you to disable roaming or forcing wifi nm-applet only allow connection to one access point and ignore the others.
Kind of flabbergasted that linux touts how powerful it is but doesn't have some of the basics ironed out.
nm-applet is still a big pain from back in 2008 when using it. Buggy unstable and still is.
I get disconnections from time to time. And nm-applet refuses to re-establish connection and have to go in and manually turn off and back on to get the so called auto-connect to work again.
Was trying out xfce and they used Wicd instead of nm-applet. Not as slick looking but it actually works reconnecting to a lost connection automatically. Thou quite a bit tad slower connecting. And doesn't support VPN or some of the wifi encryption schemes it seems.
These kind of issues had me leave linux the last time. As wifi,graphics cards and CD/Dvd burning apps so immature compared to the windows versions. And still seems a clunky unstable affair with wifi and other hardware issues like printing,sound,etc.. I am reading about the same issues now as I did then.
I know it's not all linux fault and device manufactures shoulder a lot of the blame. But as a user I am not really trying to find who is at fault so much as just get my system running stable and carefree under linux.