Exclude certain APs from roaming

[nospam gr7 com]

Hello list,

I'm wondering if there is a way to exclude specific APs from roaming?
In my case every time I boot my notebook nm connects to an AP in the 
neighbourhood which seems to be an open system with no access key 
configured. This behaviour I notice every time when any open AP is 
active. nm first tries to connect to this one an not to the last one it 
was connected. Isn't this a security lack?

Thanks in advance, Klaus