The documentation is a bit thin, but have you looked at the Overwrite
property of SVNExportArgs
?
SharpSvn.SvnClient svnclient = new SvnClient();
SvnExportArgs ex = new SvnExportArgs();
ex.Overwrite = true;
svnclient.Export(new SvnUriTarget(source), target, ex);
I haven't tested this, so I may be completely off the mark, but it seems logical.