# But I have a few things against you

"I disapprove of you because of a few things you have done" or "I am angry with you because of a few things you did." 

# who hold tightly to the teaching of Balaam, who

Possible meanings are 1) "who teach what Balaam taught; he" or 2) "who do what Balaam taught; he." 

# who taught Balak to throw a stumbling block before the sons of Israel

"who showed king Balak how to cause the people of Israel to sin" 

# be sexually immoral

"sin sexually" or "commit sexual sin"