There are various ways to specifically inhibit Wnt signaling, in cell culture and/or in animals.

1. The first choice would be to use RNAi targeting various component of the pathway, such as LRP/Arrow, Dishevelled, This has been shown to work very well for Drosophila S2 cells (Matsubayashi 2004, Cong et al, 2004) but also in mammalian cells (Lu et al, 2004).
2. There is a variety of small molecules that have been shown to either inhibit or activate Wnt signaling to various degrees. See this separate list of molecules and their targets. One of these, IWP, has been shown to be effective in blocking Wnt secretion through inhibiting porcupine (Chen et al, 2009).
3. The (secreted) Wnt signal can be blocked by an excess of the ligand binding domain of its receptor, Frizzled. This domain is best made as its natural fusion in the FRP/Frzb form. Alternatively, it can be expressed on the surface of target cells using a GPI anchor, which works well (Cadigan, 1998). It is not clear how specific the binding between various Wnts and FRP is, i.e. which FRP to use, so it is best to test a few.
4. Another way of inhibiting Wnt is to add excess of Dickkopf (Dkk) protein (Glinka, 1998). This works well in cell culture and in vivo. Dkk binds to the LRP co-receptor for Wnt. Whether all Wnt members will be blocked is not clear.
5. To block signaling inside cells, several workers have used dominant negative Dishevelled (Wallingford 2000).
6. In our own experience and in many other labs, overexpressing intact Axin, a negative regulator of the Wnt pathway, works very well (Zeng, 1997, Itoh 1998; Willert, 1999)
7. Overexpressing full length GSK can also block Wnt signaling effectively (He, 1995)
8. There are dominant negative forms of TCF that can be used to block Wnt signaling in the nucleus (Molenaar 1996).