'A new brush sweeps clean' means that someone with a new perspective can make great changes. However, the full version is 'a new brush sweeps clean, but an old brush knows the corners', which warns that experience is also a valuable thing. Sometimes 'broom' is used instead of 'brush'.
Ifs and Buts is a term used to describe the reasons people give for not wanting to do something. To show that you don't wish to accept any excuses, you can tell somebody that you wish to hear no ifs or buts Here IF & BUT have become nouns