This is simply "Disable transfer window" feature that was very often requested on steroids. :)

Most important things you need to know:

• Works only in career mode.
• Works only when you are playing with the Live Editor.
• Banned team won't be able to buy/loan-in players, but will be able to sell and do loans out.

How to ban team

• Open Team Editor as mentioned in Editing Team

• From menu bar click on Team -> Transfer Ban.

• Change Team Is Transfer Banned to Yes
• Change the transfer ban end date if you want. By default the ban should last 1 year

• Click "Save" button

If you want to ban all teams in given league then also change Ban All Teams In League to Yes

How to unban team

• Open Team Editor as mentioned in Editing Team

• From menu bar click on Team -> Transfer Ban.

• Change Team Is Transfer Banned to No
• Change the transfer ban end date if you want. By default the ban should last 1 year

• Click "Save" button

If you want to unban all teams in given league then also change Ban All Teams In League to No

If you want to ban all the teams in the game then use this lua script

LUA Functions

You can use Transfer Ban Manager to manage transfer bans via LUA

FAQ

• How can I ban the team forever?

  • Ban the team until year 2100 or something like that.

• Where the info about transfer bans is stored?

  • The file is called transfer_bans.json and it's stored in C:\FC 25 Live Editor\extensions\careers\<YOUR_SAVE_UID>