Assuming a covenant and magi not heavily specialised in hunting dangerous renegade and non-hermetic threat, I would go with darkwing suggestion: blindfold, gag, handcuffs (there is a variety called "poucière" in French, holding both thumbs thightly, making it even more difficult hand gesture).
Then strip him naked to deprieve him of any magical items or arcane connection he could have. Be suspicious about tatoo and strange marks.
I would add to keep it in small cell where it is impossible to lay, stand or sit - it should prevent the magus to recover from exhaustion, hence making spontaneous casting unlikely to happen.
Keep in mind that all those options are non-magical, hence do not need to penetrate and can be known by mundane, skilled witch-hunter.
Then, if you are in good term with the local clergy, see it is possible to keep the prisoneer on holy ground. Every bits help. Probably a simple and crude way to imprison him if there is not adequate cell, would be to bury him up to the neck in a holy ground, gag him and blind him and keep him in total silence (a PeIm around him, but not necessarly on him - so no need to Penetrate), so he has no reference and has heavy penalty on any power use and cannot know where he is.
The way I could see it done is a buried mage, neck deep, in the church backyard, gaged, with a bucket on his head and a grog sitting on the bucket. "Hello, mam' How do you do ! Me ? 'am just keeping the priest's apples from being stolen. 'ave a lovely day mam'." (yes, I have a cartoony imagination).
Finally, the last non-magical means I can think of are drugs keeping the magus drowsy, incapable of focusing or downright knockout - but you need to find a way to feed him or he would die from starvation if kept for too long.
Once you have access to magical knowledge, taking sample of arcane connection is a good step that every magus would remember to do. InVi on tatoos, marks and overall to see if despite being naked there is still something magical somewhere.
Otherwise, you need to rely on spells or magical items:
- Making all the items used to neutralised the prisonner impervious to magic (ReVi of some sorts). It could be surprisingly effective: with a magus at -15 on all his casting attempt (unless virtues like Silent Casting, deft form and such), with only one Fatigue level left, Penetration total should be quite low, making even small MR quite challenging to overcome.
- Mu spells are quite useful: most animal or plant form are incapable of speech and gesture, even movement.
- Obviously Mentem spells are useful to prevent use of magical or surpernatural ability, as long as you know what you want to mask/make him forget. Keep in mind that such spells could interfer future InMe during investigations or could be use by a clever defendant to say that his client's mind has been altered with fake memory and he is not able to discern the true event from the fake, and the magi are putting on him the blame for the actions that they did.
- In Mages of Hermes, there is one Flambeau magus who has a contraption design to prevent spell caster to use their ability. The base spell can be used to design effective nullifying spells - but this is for truly specialised magi.
- There was debate if Circle and prison wards were able to prevent the trapped creatures to use its ability - you might want to have a look at the topic and bring the discussion with your troupe to see what will be your guidelines on that one.
Because most of those spells requires to Penetrate, they are potentially less efficient than the mundane tricks.
In fact, against non-hermetic powers, magi are not well suited to efficiently prevent their magic from happening. To be trully effective, the magus designing a counter spell should know the source of power (which realm it is aligned with) and a good understanding of what it does (summoning, entrancement, curse...). So it is unlikely that generic magus are prepared the first time they encounter such issue.
Of course, after that, it all depends on the power/spell/virtue combo the imprisonned magus has access to. If he has mastered Leap of Homecoming, Turn to Mist and deft Corpus, tough luck, you've got a slippery one. By the way, if you design a diabolist to be a magus enemy, you should consider what would be his plan to escape in case of being capture, but avoid a full-proof escape artist, it will feels to much like DM fiat.
Like in most case, given enough time, even junior magi can come up with efficient ways to keep a neutralised hostile magus from escaping.