Certain keys, when pressed, will trigger Vim’s indent feature, which will attempt to set the correct amount of indentation on the current line. (You can manually trigger this by typing == in normal mode.)
You can change which keys trigger this behavior, but first you need to know what indenting mode is being used.
First, execute :set indentexpr?. If it is nonempty (I would expect this for Python), then indentexpr mode is being used. In this case, executing :set indentkeys? gives you the list of trigger keys. To remove the colon, execute :setlocal indentkeys-=:.
If indentexpr is empty, then you are probably using cindent mode, and :set cindent? will tell you that cindent is set. In this case, do the same as before, but using cinkeys instead of indentkeys. (Note that indentexpr mode takes precedence over cindent mode.)