Spikebreaker - The Rules

Telepaths, when trained to the degree where they control their abilities enough to use them, can read the surface thoughts of others. Usually they can refrain from doing so, although excited or agitated thoughts can be hard to block.
Reading surface thought can usually be done without alerting the subject, although the untrained can sometimes be detected, even if only by a vague unease.
Digging deeper into a target's thoughts is more intrusive, and the target is usually aware of what is happening (unless the telepath is particularly delicate, and usually intrudes when the target is less alert, such as through their dreams), often fighting the intrusion, even a friendly one, as an involuntary reflex.
Telepaths themselves have defences against such intrusions, but even they can be overwhelmed by more powerful or experienced telepaths, or a group of telepaths working together.
Most telepaths can also direct blasts of psychic force sufficient to stun most targets. This also causes an involuntary defensive response from the target, which comes from the same area of the brain as resists deeper scans.
Their telepathy may also be used to block, or shield, another telepaths powers, but this requires their full concentration, and can be difficult to do alone unless the target is significantly less skilled. On more capable targets, multiple telepaths would have to work together to shield them.
Some powerful and experienced telepaths can influence the thoughts of others, but this requires great delicacy to do without alerting the target, otherwise it becomes an ongoing battle of wills to get the victim to do anything, and it's unheard of for this to be successfully done to another telepath.

Spikebreaker Training
Specialized training allows SPI officers to more efficiently resist telepathic intrusions. Since they're required to spend long periods in light contact with telepaths, they're trained to keep their thoughts below the level at which telepaths can easily scan, keeping their surface thoughts blank. They're also able to better defend themselves from deeper scans, although it has been known for some to be left in vegetative states from the attack.

Screamers emit a piercing blare on the frequency telepathy operates, so any telepath who hasn't closed their abilities off will be assaulted by this barrage. The damage is usually enough to stun them for a good half hour, and they're usually unable to use their telepathic abilities again for a couple of hours.

Scanning the wearer for elevated levels of MacGuffin secretions which are triggered when resisting intrusive telepathic contact, when these are found, the baffler emits a psychic static, which interferes with incoming psychic signals.