There is no straightforward way to remove all the information of a project from the SVN repository and retrieve that diskspace. The only way to achieve this:
Dump the repository to a dumpfile.
$ svnadmin dump C:_Rep > C:_rep_dump
Filter out the project you want to remove from the dumpfile.
$ svndumpfilter exclude Project_to_del < C:_rep_dump > C:_rep_dump_updated
If you have many projects that you want to remove, repeat this step for each of them.
Create a new repository.
$ svnadmin create C:_rep_new
Load the updated dumpfile to new repository.
$ svnadmin load C:_rep_new < C:_rep_dump_updated