There is no option to prevent the reconnection. You can make the reconnection signal less “alerting” Please take a look in the Appearance section of the preferences.
To add to Walter’s reply. You should look at Preferences > Appearance > Specify reconnection display:
Display as Panel will act like it does now, will popup the roster window. The other two options are silient. You may choose one of them. This will work in 2.6.* versions of Spark.