There are two ways that you can remove multiple contacts from a Custom tag* at once, either:

• Remove multiple contacts from a Tag within the Tags menu.
• Or remove multiple contacts from a Tag within the Contacts menu.

* Integrated, Intent and Shared tags cannot be removed.
You can also also remove custom tags from individual contacts; Add multiple contacts to a custom tag; or delete custom tags from your account. 

Remove multiple contacts from a Tag within the Tags menu

Start by navigating to the Tags menu.
From here:

• Locate and select the tag that you would like to remove the contacts from.*
• Within the list of contacts, use the check boxes to select multiple individual contacts, or tick the Select all box.
• From the Bulk Actions, select Remove Contacts from Tag

• Select Confirm to remove the contacts from the tag.

Remove multiple contacts from a Tag within the Contacts menu

Start by navigating to the Contacts menu.
From here:

• Select the Contact List tab.
• Use the tick boxes to select contacts, either by:
  
  • Ticking individual contacts;
  • Using filters to define a group of contacts;
  • Holding down shift + click to select all contacts in a sequential order;
  • Selecting all contacts from the current page;
  • Selecting all contacts in your account.
• Once the contacts are selected, from the Bulk Actions options, choose Remove tags (1).

• In the subsequent options, start typing in the tag you want to remove. Matching tags will be suggested in a dropdown. Click on the relevant tag to select it.
• Repeat this if you would like to remove multiple tags.
• Select Remove Tags

* If a selected contact does not have the tag, this will not be changed.