Short answer - just comment it out or delete, Debian has put lxqt into it's list of projects.
In general irc on freenode lxde is the source of quick answers to all things lxqt. And listed there is other lxqt web sites. irc on OFTC lxde is the irc channel primarily for lxde.
But for this particular answer - one of the major venues of the freshest lxqt binaries was via Siduction (that is a Debian variant based on sid - where things get broken.) Alf Gaida was/is the main contact. However Debian in its wisdom (no sarcasm) has started to funnel the newest lxqt binaries straight into Debian via Herr Gaida. If you stay with testing, you will get it, but a bit slower presently with the Debian freeze.
So you can just comment out the siduction line or remove entirely. If you really have to look at and use the newest developments you could set up a virtual sid insttall. I do have one. But when testing unfreezes, Sparky will get the newest binaries of lxqt about 5 days after the freeze ends. Life is good. Let the people dealing with sid squash many of the bugs first and profit. Peace out.
The stable release of LXQt is available in Debian's official repositories as of Debian 9 "stretch" which represents the testing distribution as of 11/2015. There are metapackages lxqt-core providing core features and lxqt providing LXQt as a whole plus some additional applications.
Siduction used to provide repositories which could be used on Debian as well earlier but these have been abandoned in favour of the official Debian repositories.