Toggle Tag
You can use the Toggle Tag plug-in to toggle a specific tag for the selected item. If this tag is part of a group of tags, Toggle Tag can easily be configured to remove any other tags in that group automatically.
For example, if you had a Priority group of tags containing Low, Medium, and High, you could create Toggle Tag plug-ins that set the tag to Low, Medium and High, ensuring that only one is applied simultaneously. This is especially convenient when you assign keyboard shortcuts to these plug-ins.
Generate Toggle Tag
If you have a lot of tags to toggle, check out the Generate Toggle Tag plug-in. It adds a submenu to the Automation menu that includes all the tags you select. You can optionally specify that only one tag should be applied at a time within a group of tags.
For example, you could have a group of Weekday tags that contains a tag for each day of the week and use this plug-in to create an automation that allows you to apply one (and only) of these tags to the selected projects and actions.