I believe you are asking for options that will help you to show only spellcheck in the Rich Text Editor,
The option Neha suggested might help you to display the spell check option, but it will still show the default icons of Bold. Italics, Underline, Font family and size, etc. and I dont think there is a way to disable them.
Pega 7 RTE control is built on top of CK Editor 4.3x. If you have to show only spell check icon then you may have to modify the internal scripts (like RULE-FILE-TEXT WEBWB PZPEGA_CKEDITOR_EXTRAS!JS). I would not recommend that you do this as this could have potential side effects.