To cite spatstat in publications use:

Adrian Baddeley, Rolf Turner (2005). spatstat: An R Package for Analyzing Spatial Point Patterns. Journal of Statistical Software 12(6), 1-42. URL http://www.jstatsoft.org/v12/i06/.

If you use hybrid models, please also cite:

Adrian Baddeley, Rolf Turner, Jorge Mateu, Andrew Bevan (2013). Hybrids of Gibbs Point Process Models and Their Implementation. Journal of Statistical Software, 55(11), 1-43. URL http://www.jstatsoft.org/v55/i11/.

Corresponding BibTeX entries:

  @Article{,
    title = {{spatstat}: An {R} Package for Analyzing Spatial Point
      Patterns},
    author = {Adrian Baddeley and Rolf Turner},
    journal = {Journal of Statistical Software},
    year = {2005},
    volume = {12},
    number = {6},
    pages = {1--42},
    url = {http://www.jstatsoft.org/v12/i06/},
  }
  @Article{,
    title = {Hybrids of Gibbs Point Process Models and Their
      Implementation},
    author = {Adrian Baddeley and Rolf Turner and Jorge Mateu and
      Andrew Bevan},
    journal = {Journal of Statistical Software},
    year = {2013},
    volume = {55},
    number = {11},
    pages = {1--43},
    url = {http://www.jstatsoft.org/v55/i11/},
  }