As I know, one mainly way is taking continuous subculture of your primary culture or plunge your explant into a common household bleach, and then wash out with distilated water and put onto a medium agar.

For seeds you could use the commercial fungicyde mixture to treat  seeds before sowing into the soil

We have used for many years a chlorine-gaseous producing system derived from the interaction of one volume (50 ml) of Clorox (commercial sodium hypochloride) and one volume (50 ml) of HCl 6N for at least 30 minutes in a close container and of course in an extraction cabinet. Since hypochloride gaseous are derived from this mixture, you have to put this solution in the base of the container and your explants must be separated physically from this reaction, in the close container. You can put the explant in a petri dish and once finished you open yor container and being carefull of gaseous emanation, close the petri dish, take your explants and let them to liberate chlorine gas for a while (1 hour), after that your seeds are already sterilized and free of contamination. We have used this protocol for many species, like arabidiopsis, corn, rice, sorghum, wheat, etc. It works perfectly specially from seeds derived from greenhouse.

This a view of the system. Good luck

you can use a solution of Plant Preservative Mixture (PPM),  put the seeds in a solution of 4-5% v/v of PPM for 12 hours.

or with seeds probably a thermic control may function (if the seeds ressist it); put the seeds in a oven or a freezer 6 to 24 hours. You must check the viability of the seeds with tetrazolium. 

i think you can use Bavistin its a fungicide to minimize the contamination before surface sterilization. The seeds were dipped in bavistin for 5-7 min. and then washed under running tap water...