Giving up on something

Tirar la toalla

tee-RAR lah toh-AH-yah — To throw in the towel

"Después de tres intentos, decidió tirar la toalla."
After three tries, he decided to throw in the towel.

Comes from boxing, exactly like the English idiom — one of the easier idioms to remember because it maps directly onto English.

"Something I refuse to do with a stubborn student — we always get there eventually."— Graciela