Simply open the messenger window by clicking on the lightbulb icon in DeskNow.

Then click on the 'Options' icon (a pointing finger) and uncheck the option 'Show alert when users get online'.

The default option for all users can be set (unless they have explicitly enabled/disabled it), in the Administration/Instant messaging page.

To disable the option even for those users that have already enabled it:

run the following query on your database:

update DUSER_PREFERENCES set pvalue=replace(pvalue,'<presenceAlertsEnabled>true</presenceAlertsEnabled>','<presenceAlertsEnabled>false</presenceAlertsEnabled>') where groupname='XMPPStoredProperties';

then restart DeskNow.