If your are using flake8 as a python syntax checker you could do it like this (put it into your vimrc or ftplugin/python.vim file):
let g:syntastic_python_checkers=['flake8']
let g:syntastic_python_flake8_args='--ignore=E501,E225'
You need to silence each error class explicitly (and cannot disable pep8 checking as a whole). See the flake8 documentation and pycodestyle documentation (used to be pep8) for all error and warning codes.