Relentless Avenger too OP?

For a level 7 ability this is too broad, it has no save nor (more importantly) size restriction.

I had a situation where an adult dragon had to flee, but I had no justification to make it resist the ability...imagine if they were fighing Tiamat!

I like to overrule as much as the next DM, but it feels really scummy to not follow rules as written.

Level 7: Relentless Avenger

Your supernatural focus helps you close off a foe’s retreat. When you hit a creature with an Opportunity Attack, you can reduce the creature’s Speed to 0 until the end of the current turn. You can then move up to half your Speed as part of the same Reaction. This movement doesn’t provoke Opportunity Attacks.