Antonyms for the phrase "signs up for" include "cancels," "withdraws," "avoids," "declines," and "refuses." These words convey the opposite of officially registering for or committing to an activity, event, or service. For instance, if someone were to decline an invitation or refuse to sign up for a membership, it would indicate that they are not interested in partaking in said activities. Similarly, if someone were to cancel a reservation or withdraw their application, it would imply that they are no longer interested or able to attend the activity or service. Antonyms for "signs up for" signify a marked absence of willingness, motivation, or enthusiasm.