To cite spa in publications use:

Mark Culp (2011). spa: A Semi-Supervised R Package for Semi-Parametric Graph-Based Estimation. Journal of Statistical Software, 40(10), 1-29. URL http://www.jstatsoft.org/v40/i10/.

Corresponding BibTeX entry:

  @Article{,
    title = {{spa}: A Semi-Supervised {R} Package for Semi-Parametric
      Graph-Based Estimation},
    author = {Mark Culp},
    journal = {Journal of Statistical Software},
    year = {2011},
    volume = {40},
    number = {10},
    pages = {1--29},
    url = {http://www.jstatsoft.org/v40/i10/},
  }