If you require them to sign up to get the free trial then you can allow each account to only have 1 free trial. Then either have the free trial use the account

If you require them to sign up to get the free trial then you can allow each account to only have 1 free trial. Then either have the free trial use the account to activate or simple only let each user download once and have the installer delete itself after installation. They will need another email address and account to get another free trial so I don't think many people would bother.