There is no easy way to disable Sitecore workflow.
You could remove the workflow item with subitems (workflow states, commands, actions) and select
Remove Links options when Sitecore warns you that there are some items which link to the items you want to remove.
But I assume there are multiple items in
Waiting for Approval and
Approved state. What should happen with them?
Approved items removing both workflow and workflow state shouldn't matter - they are approved anyway and published, right?
For items in
Draft state and
Waiting for approval state if you remove workflow and workflow state you may end up with publishing items which are not ready to go public.
I would suggest to remove the workflow from the
__Standard Values items for any templates which use the workflow now. That would make sure that no new item will be added to any workflow state. Then for every item which is in
Waiting for approval state you should ask content authors to either approve them and publish them, or to remove the version which should never go public.
After all items are in
Approved state, you can remove the workflow with its states.
If removing the workflow from
__Standard Values items will remove it from all the items which use that template, remove the workflow from
__Standard Values only after there are no items in
Waiting for approval state.