Its easy to remove the App completely from your theme
Follow these Steps:
1) How to Uninstall App in Shopify
Step 1: Go to Settings, then go to Apps. Log in your store with the store owner's account, then go to the Settings field in the bottom right. ...
Step 2: Click on the "Apps & Sales Channel Tab" from the settings page
Step 3: Find "Ecomartists - Drawing on Demand" App and click "Uninstall"
2) Delete Script Tags
Step 1: Click "Online Store" on sidebar
Step 2: Choose themes
Step 3: Find your Published Theme and click "Edit Code"
Step 4: Under "Snippets" Find "ecomartists-upload.liquid"
Step 5: Press the trash can to delete
Step 6: Find "theme.liquid" under layout section
Step 7: Scroll to the bottom and find "{% include 'ecomartists-upload' %}"
Step 8: Delete and Save theme.liquid
Congrats you have successfully removed Ecomartists app and all its components.