This petition was rejected

The Government e-Petitions Team gave the following reason:

We can't accept your petition because the UK Government and Parliament aren't responsible for the issue you raise. Deciding whether to put someone in prison is the responsibility of the courts, not the UK Government and Parliament.

It's already a criminal offence under the Terrorism Act 2000 to belong, or profess to belong, to a proscribed organisation like ISIS in the UK or overseas.

If you would like Parliament to ban ISIS fighters from returning to the UK, you might like to sign this petition instead:

petition.parliament.uk/petitions/204079