Once the Quote status changes to accepted or denied I want to make sure that it won't be allowed to change again. Can it be done with flow? Any suggestions how can I make that happen?
You can do it using a validation rule:
AND( ISCHANGED(Status), OR( (TEXT( PRIORVALUE(Status)) = "Accepted"), (TEXT( PRIORVALUE(Status)) = "Denied") ) )
The ISCHANGED function evaluate to true when the update on the Quote involve the field "Status". The PRIORVALUE function returns the value of the Status before the update.
Basically the whole formula evaluates to true if the status is changed to any value, and the previous value was Accepted or Denied.