Find the .hgignore file and open in a texteditor
Add a line e.g. myfolder/mysubfolder/myfile.txt
or for a for the contents of a folder e.g. myfolder/mysubfolder/dontinclude/
If you already have a file in source control and you want to remove it, open a command prompt, navigate to the directory.
hg remove -Af myfile.txt
-A records a delete for missing files
-f forces this
So you could probably omit these switches.
Then commit and push as per usual