Random people start downloading from me and I can't stop/cancel their download?

I allow only people in my list to download files from me and in the old program there is about button .. but now with this "QT" program I can't find out how to do that.

Sharing files privately is not possible with SoulSeek QT.

The only users you may decide wether or not to share files with are those who are not sharing anything themselves. This option can be set on Options > File sharing > Send warning message to downloading user with empty shares.

If you want to share files privately, you ought to use SoulSeek NS.
The latest version of SoulSeek NS can be downloaded in the following link:
http://www.soulseekqt.net/news/node/683

If using SoulSeek NS, SoulSeek QT users will not be able to download your files if your options are set to do so.

Note that this version is no longer in development.

-Jeff

SoulseekQt does let you share privately. When sharing a folder, you can uncheck 'Folder is accessible to everyone', which will leave 'Folder accessible to all user list users' checked by default. You can even uncheck that and select specific user groups to share the folder with, though you will have to define some user groups via the 'Set user's groups' context menu option first.