How can I restrict tags that are able to be used?
That’s not possible, as tags are meant to be more of a “folksonomy” rather than a strict taxonomy.

Sounds to me like categories would be a better fit for your needs. With those, the options are defined in advance, and chosen from the fields, rather than created from the fields.

Update:

We just updated the Categories and Tags feature page with a new video that may do a better job showing the difference between the two.