Learn English – slang word or idiom for someone who borrows money from friends or relatives and never (or rarely) pays them back

The following, although not slang, may be relevant:

• practical joker, “someone who instigates practical jokes” – wiktionary
• prankster, “One who performs pranks” – wiktionary
• trickster, “One who performs a trick”; also “A mythological figure responsible for teaching others through the use of guile and treason” – wiktionary

Wikipedia's practical joke article says the following:

A practical joke (also known as a prank, gag, jape or shenanigan) is a mischievous trick or joke played on someone, generally causing the victim to experience embarrassment, perplexity, confusion, or discomfort. Practical jokes range from confidence tricks [to] hoaxes ... Practical jokes or pranks are generally lighthearted, reversible and non-permanent, and aim to make the victim feel foolish or victimised to a degree, but may also involve cruelty verging on bullying if performed without appropriate finesse. [emph. added]

The classic idiom in English is "third wheel". This can be used both against both parties (the couple and the third party). For example:

The couple might say: "He is such a third wheel. I wish he'd stop trying to hang out with us."

The third party might say: "They make me feel like such a third wheel. I don't understand why they invite me along when they clearly don't want to hang out with me."

A more pejorative (and much more vulgar) term might be "cockblock". The implication here is that the intruder is preventing the couple from "getting it on" (having sex) by blocking the male's cock (penis).

"Tommy is such a cockblock. He needs to get a life."

However, cockblock isn't necessarily only about stopping sex. In fact, cockblock has been expanded to include any individual that stops something that people want to do, especially in online gaming:

"Tommy is such a cockblock. I almost got to that extra-ammo pickup, but he killed me before I could. He needs to get a life."