Hi @Zango,
It is quite common to amplify host DNA sequence. As you may be aware plants also have the ITS gene. In fact, there is even a reference database available: PLANiTS.
To alleviate this issue, you can try a series of different ITS primer sets that are less likely to amplify plant ITS. I do not know if this has been evaluated already, so you'll have to search the existing literature. Or someone else can let us know. 
Another option is to use a blocking primer or Peptide Nucleic Acid (PNA), to limit the amplification of your host DNA.
Otherwise you can read through the Filtering tutorials and remove any taxonomic groups you're not interested in.
-Cheers!
-Mike