How to mark a blocked street for cars

How best to represent this depends on how the street is actually blocked.

Here is one example where the footpath/sidewalk along the main road is raised, but there's a join to the old road (closed off to prevent it being used as a rat-run). Here the implicit access tags on the "highway=footway" prevent vehicular access (in your case you'd want to pick an appropriate highway tag and add "bicycle=yes" if bicycle access isn't implicit.

There's a slightly different example here. In this example there road's still there, but there's something large preventing vehicular access (actually a large planter rather than a bollard). There are also explicit access signs on a short stretch of road, which have been mapped.

So to sum up:

1. Split the road as required.
2. Choose an appropriate highway tag for each section, if necessary.
3. Add extra access tags, if necessary.
4. Add any explicit obstructions (e.g. bollards).

A search of the Wiki (documentation) using altenativly barrier or bollard should answer your question.