Restricting property instances in OWL

Hi,

I am trying to define an OWL ontology that lets me do the following.
Consider that I have an Edge which connects two Nodes. However, I wish to
say that the Edge source is not the same as the Edge target, i.e. the Edge
does not connect the same Node to itself.

Is this possible to do in OWL, i.e. define that "source != target"? It seems
that the restrictions apply only to classes and data types, not to
instances.

I have also thought of having
Node <- EdgeSource
Node <- EdgeTarget (these two classifications are disjoint)
Edge has property "source : EdgeSource" and "target : EdgeTarget".

This would make sure that the same node cannot be connected, but also in
this case the same node could not simultaneously be the source and the
target of two distinct Edges.

Any assistance or ideas would be appreciated. I am also look at rules
through OWL+SWRL.

Thanks
Jevon