I personally will avoid their IMs/phone calls and take more than half a day to reply the text (sometimes I'll ignore text). If they insist on inviting me to something, I'd tell them I am swamped at work. I usually don't lie about having a boyfriend. Would you do that? Lie about having a bf/gf, seeing someone else, etc.? I can't keep a straight face and I fluster when asked about details.
Do you think it would hurt the other person less by just saying you are with someone else or do you insist on saying you are not interested?