If you are on Jilt's paid plan, you can remove "Powered by Jilt" from appearing in the footer of your emails. To do so
In the Jilt admin area go to Settings > Shop Settings
Uncheck 'Remove "Powered by Jilt" from email footers'
Click Update Jilt Settings
Can I remove "Powered by Jilt" if I'm on the free plan?
No, this can only be removed for accounts that are on the paid plan.