Modifying ... Help please !

Hi,

I was hoping to use this as voting system for union members, 1 vote only per member based on username. As it appears to be based on IP address if one member votes and then logs off and then second member logs in it will not allow them to vote due to using the same computer!
Can i set it to check against username rather than IP address and if so please can you show me what code I need to change as I am totally new to joomla.

Also is there anyway of only allowing say the administrator to create a poll. I appreciate this is a 'community poll' plug in but I do not wish registered users to be able to create a poll, only the site administrator.

Many thanks,

Darren.