How can I prevent extra usage of Machine Translation characters?

Depending on how you use Machine Translation, you might use more characters than intended.

If Machine Translation is added:
1. As Cloud-based Resources on projects - this is also used during Pre-translate stages, if the option to Apply machine translation if no match is found is active. 
https://docs.rws.com/813470/302731/trados-studio-2021-sr2/specifying-pre-translate-settings-for-mt-providers
To prevent extra usage, you should deactivate the Cloud-based Resources provider once a project has been pre-translated. 

2. Using the Language Weaver plug-in - if Re-send draft and translated segments option is checked.

https://community.rws.com/product-groups/trados-portfolio/rws-appstore/w/wiki/3356/language-weaver-former-sdl-machine-translation-cloud-beglobal

Basically, Trados Studio will search the Machine Translation provider as a TM if it is active and enabled, every time you place the cursor in the target segment, and so it can use more characters.
Ideally, once you pre-translated your file with Machine Translation, you can deactivate it from the project settings to prevent further usage of it.

Or you can translate segment by segment, but make sure to confirm the segment (Ctrl + Enter). Confirmed segments are no longer sent to the NMT servers for translation.