Go to your whoisonline template and find
Code:
<if condition="$show['ip']"><td class="thead">$vbphrase[ip_address]</td></if>
and replace it with
Code:
<if condition="$bbuserinfo['usergroupid'] == '6'"><td class="thead">$vbphrase[ip_address]</td></if>
This should remove the entire IP field for all except Admin. If your Admin usergroup is not 6 change it in the second line of code.