Go 5 man premade, all pick TP summoner spell, go all mid, give free kills, go afk and watch how enemy team will finish at 7th minute mark, HA-HA, so fun ...
Because that happened before and is happening again I think your warning (banning some of people who did that) wasn't enough, they're still doing it, something needs to be done.
You should've make some rules like:
- if you have less than 2k gold at 10th minute you don't receive any IP at all (you passively get approximately 1500 gold in 10 minutes, this is amount of gold without any cs and kills including starting gold, I tested it myself in custom few minutes ago)
- You don't get any IP for match that lasts less than 15 minutes (instead of 7 minutes), bots are not included in this rule.
I don't know what you think, but I don't think any game that lasts less than 15 minutes can be fun for both sides, it is more likely a very one-sided match.